I did this a couple months ago and went through a similar research process. I installed speakers in a 95 RMS so should be similar. I researched the topic on the forum and there was generally good advice. There is also a good thread on stationwagonforums here:
Such a Deal!?!?!? "Garage Find" '92 Roadmaster Sedan | Page 4 | Station Wagon Forums which provides detailed pictures on removing the driver side panel. The whole thread is a great read but the door panel is about half way down that page. His is a 92 so some things will be different.
Car Stereo wiring colors I used:
1991 Buick Roadmaster Car Stereo Wire Colors and Locations
I used 6.5s, specifically these:
Pioneer TS-600M, 6-1/2" 4-way coaxial speakers, 320W max power - Walmart.com to replace the main speaker. They fit well. Those comes with extra plastic junk either for looks or for size adapters for different cars but I think I just bolted the speaker on and re-used some sort of screen foam thing that was already in the door. There wasn't room to put them on. I did use the screws that came with the new speakers, I did not reuse the old ones. I left the tweeters plugged in. They are not worth removing and I couldn't find a reasonable replacement. If yours are blown or are crackling I would probably just cut the wires and go without.
The Christmas Tree door clips were already broken on my doors so I didn't have to pry them off, but I did need to replace them and I would suggest doing so. I used these and they worked well:
Amazon.com: Lantee Car Clips for GM - 50 Front Door Trim Panel Retainers Clips Replaces for GM 10153057: Automotive
One or two of the slots they fit into were broken beyond repair. I think I was able to wedge one in so it didn't fall out upon repair. The other I either let loose or JB welded in there, I forget. If I glued it in there I may regret that decision later.
Sorry if you know this already but I did not: You have to cut a small wedge out of the cap of the tree and use that opening to twist it in there. I was trying to force it in there like a Neanderthal when I looked at an old one and realized that it twisted in there only after doing this.
The screws in the bottom right side of my door panels didn't catch any more so I had to dig through my pile of screws and come up with a bigger one. I had to drill a slightly larger hole into the door and it was a PITA but the door panels are now firm.
WHILE YOU'RE IN THERE, you should at least inspect and grease the window rollers. Many would say to just straight up replace them but I did not. There are two per window. I had to replace a broken one. There are threads here, I think even stickied, on how to do it. I ended up using a pair of channel locks to squeeze in the new roller. Some suggest a C Clamp but I did not have one that size and couldn't really grok how to get it to fit even if I did. I did grind down the roller a bit and that made it easier to get in. These are what I used:
Amazon.com: Dorman 74444 Window Regulator Roller for Select Models: Automotive
The rear speakers 6x9 are pretty straight forward but it does involve taking out the whole back seat and stuff. Check the third brake light while you're in there. You'll need a good sized TORX socket to get the seat belts off. I think its 45 but I don't remember. I had to get a set at an AutoZone, Walmart didn't have one that big.
The head unit should also be straight forward. My 95 RMS is a double din but I've see reports of 1.5 dins on some models. I used a single din pioneer, an appropriately labeled GM wire harness convertor and one of those plastic double din to single din conversion kits that adds a pocket.
I only had two issues with the head unit install. First was that I originally bought the cheapest one I could find just to get some music in there but it did not produce any sound. Turned on, changed channels, inputs, etc just no sound. Tried troubleshooting it for a day and nothing worked. Bought a brand name one and it worked without a problem.
Second is that it is a tight squeeze with all the wiring if you get one with a cd player. I had to pull it in and out several times to make sure it got in there without crunching anything. Just a blue tooth/digital one should be a fraction of the size and would have lots of room.
Good luck and remember to be gentle with the door panels!
