A 91 should be pre-wired for a Becker Silverstone CD changer. You must really love Beckers to put up with one of those - won't recognize some store CDs, won't recognize most home burned CDs, about a minute to skip to another CD. Must have been great back in 91 but not today.
Check with Becker USA on availablility of rebuilt units. I doubt you'll find any new. I believe they're $5-600 from Becker.
You'll have to scrounge around for factory/dealer brackets and the carpeted cover if you're going for that fully original look.
In my case a Sony RF changer works well enough.