Since you have to ask you most likely don't have rear speakers installed. If they had been installed you would find them (recessed mounted) behind their respective grilles under each of the side windows.
The front speakers are 4" and the rear speakers are 5 1/4 "
Both dash and rear panels use 4x6 in speakers
on the outboard side near the lugs.
It is inside the passenger side rear wheel well.
According to crutchfield.com my 1995 Chevy c/k 1500 extended cab pickup has 4"x6" speakers in the rear pillar but you can fit a 4" with a bracket of course
From what I have heard from others it is located in the rear diff. That is the best I have for you though.
most usually there was a dealer kit that replaces the factory rear side storage box doors to install rear speakers , although only in xtra-cab pickups.
there is asensor on the rear diff...replace that and you should be ok
6 1/2"
Dashboard are 4x6 rear corner are 4x10
on the passager side just in front of the rear wheel. behind one of the bed supports
; In-dash stereo : 5-13/32" depth ; Front speakers : Dash: 4x6" : Front Door: 6-1/2" factory speaker note ; Far rear speakers : Rear Pillar: 4x10"