Looking to replace the failed bose system. I realize I will have to run new wiring. My question is what stereo/head unit will fit into the opening so I dont have to butcher the dash face or it stand out like a sore thumb? Any recommendations?

When I replaced mine, I bought everything I needed from Crutchfield.  I was told to make sure to disconnect the wiring harness to the bose relay to avoid battery drain problems.  On '89 it's up behind the breadbox, but can be reached through a hole while the stereo and heat controls are out.  I just left the relay where it was, to avoid a lot more dash disassembly.  I used the orig. +/_ speaker wiring (they go back to the stereo, not the relay)  Once I got the stereo wired into the factory harness, the only thing that doesn't work like orig. the stereo lights won't dim via the headlight switch (may be different with a different deck)  Another thing I read, if your new deck is long (deep?) as in CD player (mine's short, all digital) you may have to trim some plastic in the back of the hole.  It took me a while to get it all figured out, but I've owned a lot of cars and installed a lot of stereos.  I have no idea why the pic turned downside up, but you can see at the bottom (top) where I put the mic so I can make phone calls.  Opinions vary, but I don't consider my thumb sore enough to worry about.

Looks like you installed it upside down looking at the picture 😃

Thanks! Yes, I was actually wondering if the original wiring to the speakers could be used. I am replacing in a 1990 ZR1, dash is a bit different. I would think the wiring is probably fairly much the same and in a similar location. Thanks for the response!

You don't want to use the existing wiring. Long story short - the present wiring is only delivering line-level signals, the amps themselves are located at the speakers - these wires will not support a good amped head unit with good speakers.  Save and secure the present wiring (in case you want to return back to OEM equipment) and run new speaker wiring (at least 16 gauge, depending on setup).

I've done about a dozen installs on later model C4s. Kenwood and JVC head units seem to fit a little better than Pioneer, but they all will fit with a little trimming. 

Actually, when I talked to Crutchfield tech support, they said as long as I didn't drop in a power amp and go over 100 watts RMS per channel, the factory wire was more than sufficient to run my JVC deck.  It's been in over a year now and no problems.

thanks!

Are you going to do the install or having a shop do it? Either way, do some research on the "SYS" issue that will arise after replacing the head unit. A couple of resistors will fix it, just have to know where to put 'em.