The dash speaker is and odd size.  Something like 4x10 if I remember right.  To get the grille off, insert a thin screwdriver or pocket knife under the upper corner of the grille and pry the grille up.  It has three tabs that stick in holes on the bottom but the top just has a little lip that makes and interference fit and is easy to overcome.  I personally would not use a center speaker if you are going to use the door speakers.  It would really mess up the stereo separation to use a mono speaker in the center and to the best of my knowledge, they were only used with the base radio that only came with one speaker in the earlier models.  I mainly use that speaker grille to route wires up to my GPS and other accessories mounted on the dash.

 

On a side note, one thing I did that was real simple was mount a couple extra 6x9 housings in behind the front seats. These are the typical 6x9 housings you can get at Walmart or anywhere else. The fit good against the side of the cab and the way mine have recessed side panels they actually make a handy place for backseat passengers to sit stuff. Here is a pic:

Naturally that won't work if you have the full interior panels in the rear but I have yet to own a Ramcharger with the full interior, lol.

 

The dash speaker is and odd size. Something like 4x10 if I remember right. To get the grille off, insert a thin screwdriver or pocket knife under the upper corner of the grille and pry the grille up. It has three tabs that stick in holes on the bottom but the top just has a little lip that makes and interference fit and is easy to overcome. I personally would not use a center speaker if you are going to use the door speakers. It would really mess up the stereo separation to use a mono speaker in the center and to the best of my knowledge, they were only used with the base radio that only came with one speaker in the earlier models. I mainly use that speaker grille to route wires up to my GPS and other accessories mounted on the dash.

Yep Door Speakers were used on Stereo models, the Center Dash Speaker was used on Mono models

And it is a weird size like Elwenil mentioned.

Once upon a time in the Junkyard i saw a Swinger that had 2 Speakers in the Dash on outside edges of the Defroster Vents. It looked factory as in you couldn't see any kind of modification, i pulled the Dash Pad out of it (Mint condition White Dashpad) for my Demon and underneath/inside the dash it also looked original. First time i'd ever seen 2 Speakers in the Dash on an A-Body. Either it was one hell of a good custom job, or it was an option i've never seen/heard of before

 

Yea the factory center speaker is a 4x10 like alot of GM vehicles used.  Factory door speakers are 6x9's.  Some of the LE trim 90's models trucks had 4 speaker systems with 4x10s mounted in the rear pillars of the cabs.

My truck came from the factory with an AM radio with the center speaker.  It was all rotted out from sun rays so i replaced it with one of those $10 no name stock replacement speakers you can by at Wal-Mart individually instead of in pairs.  Went to Radio Shack and bought a cable with a 1/8" mono headphone jack on one end and soldered it to the speaker.  I now have one hell of an external speaker for my CB radio that doesn't take up any dash space.

When I installed a CD player in my truck, I cut the doors to fit 6x9's

 

Yep - dash speaker was not used (empty) on am/fm cassette stereo models.

 

Fwiw, when put in my cd player I took out the dash speakers, there were two 4" speakers in there. Looked like factory speakers too, cheap paper cone with no name on them. So idk if it was possible to have them from the factory or not. Doesn't matter really, now it's just an empty hole in my dash, that is unless I get an in dash DVD player, then I'll replace the center speaker for the 5.1 surround sound.

 

On a side note, one thing I did that was real simple was mount a couple extra 6x9 housings in behind the front seats. These are the typical 6x9 housings you can get at Walmart or anywhere else. The fit good against the side of the cab and the way mine have recessed side panels they actually make a handy place for backseat passengers to sit stuff. Here is a pic:

Naturally that won't work if you have the full interior panels in the rear but I have yet to own a Ramcharger with the full interior, lol.

This is actually about the location of the rear speakers on later RCs (1990 -1993 I believe) with the full panels in back. They had 6x9s mounted in about that location in the side panels. So if he has side panels, best bet is to cut holes to mount some 6x9s. With full panels, there's even just enough room to mount a small amp on the side wall for a little more power.

 

Fwiw, when put in my cd player I took out the dash speakers, there were two 4" speakers in there. Looked like factory speakers too, cheap paper cone with no name on them.

Later models do list a "With 2 Speakers" option, but it doesn't show a pic or anything.

This is actually about the location of the rear speakers on later RCs (1990 -1993 I believe) with the full panels in back. They had 6x9s mounted in about that location in the side panels. So if he has side panels, best bet is to cut holes to mount some 6x9s. With full panels, there's even just enough room to mount a small amp on the side wall for a little more power.

Does anyone have an pic of these rear trim panels with speakers? I thought they existed once before but none of the Parts Manuals list them and the only thing added on them are the lights and the ashtrays in the rear as far as they show. The only rear speakers listed in the manuals are for regular and Club Cab trucks.

 

my 82 has them ....about destroyed but they are there i am pretty sure lol ...i know the 87 i had definitely had them in the panel seems like the 85 and newer RC 's had the panels i know the 70's models did not even up to early 80s didnt

 

my 89 rc has the side panels lights and ash tray but no speakers i think i only have the two door speakers...which i need to replace cause the radio doesent sound its best anymore

 

my 82 has them ....about destroyed but they are there i am pretty sure lol ...i know the 87 i had definitely had them in the panel seems like the 85 and newer RC 's had the panels i know the 70's models did not even up to early 80s didnt

They were likely added on your 82.

I'll double check some of my FSMs this evening and see if I can track down what year they started, but I'm thinking 1990 was the first year for factory speakers in the rear panels.

 

Ok, after looking a little closer there is a listing for '90-'93 models with a rear speaker for D4,D8 models with a grille and a separate number for a mounting plate.  It's a bit confusing since it has several listings like "ClubCab" and "exc. ClubCab" but they both have the same part number.  The interior panel does not list anything about speakers so they must just cut a hole in the plain panel to mount them.

 

Ok, after looking a little closer there is a listing for '90-'93 models with a rear speaker for D4,D8 models with a grille and a separate number for a mounting plate. It's a bit confusing since it has several listings like "ClubCab" and "exc. ClubCab" but they both have the same part number. The interior panel does not list anything about speakers so they must just cut a hole in the plain panel to mount them.

I don't have my 90 RC anymore (wish I did), but if I recall correctly, there is a little plastic housing that goes behind the speaker (similar to the one used in the doors). On my 87, I added side panels back there and just simply cut holes in them for speakers. The speaker magnet helps 'stick' the speaker in place (not ideal, I know). My 83 had rear panels, so I once again added the speakers to them. Just enough room to add a 50 watt amp just below the speaker.