New speakers in my pickup

I have a 92 scottsdale std cab that is in need of speakers(stock ones on those older pickups sound terrible). To start off, when i first started this project all speakers except the rear passenger one worked and now only the rear drivers side one works so my first question is what do you think could have caused that? Electrical issues? wire connections? I dont think they are "blown" because there is no sound coming from them at all and if they were blown they would have sound still but the quality would be terrible is that right? Now onto the new speakers I was thinking crutchfield is the best place to get em from yes? Then my final question is installation related, I know how to install the new ones into the dash thats simple but the two rear ones im not quite sure on. How do you get to them since that whole plastic pannel covers them up i dont see any screws to remove the pannels or anything so how do those pannels pop off so i can access the speakers?

When I replaced the speakers in my 98 I remember I had to take the headliner out, then it was a matter of prying the side panels off of clips.