I'm gonna try and get some pics for of my setup and maybe even do an article writeup because a lot of people seem to be misinformed on replacing the factory stereo.
(All of this pertains to my 92 Eldorado non-bose system)
Removal is simple:
http://carstereohelp.com/stereoremovalCadEldSeville.htm
Contrary to popular belief...
There is a mounting kit available.
Here is the one I used:
http://www.partsexpress.com/pe/showd..._ID=6607&DID=7
This kit allows you to go from double DIN to a single DIN unit, and it works perfectly.
There is a wiring harness
The one I used is the one made for '88 and up GM cars.
Much like this one:
http://www.scosche.com/scosche/connectors/GM02.asp
Here is where it gets tricky. The "head unit" in the dash isn't really the head unit, it's just a tuner with a tape deck, I discovered this after I unplugged all connections to it and couldn't stop laughing because the radio was still on. The actual head unit is located in the trunk on the back side of the seats (at least in my particular vehicle). Simply pull away the trim piece that covers the speaker and place it aside. Now it's hard to explain which item is the head unit because I found many electrical boxes/device back here. An easy way is to simply find the harnesses that match and plug into the aftermarket one from above, there should 3 harnesses and 14 wires all together that.
Once you've located and matched up your harnesses you now have to run 12 or 14 wires to the front, where you will be mounting your aftermarket unit. I say 12 or 14 because 2 of the wires are for dimming, my unit does have a dimming feature, or at least on that dims with the car, i.e. when you turn the headlights on and so forth. I ran all 12 along the passenger side, down along the rear seat, underneath the rocker trim and then under the passenger seat an into the center console. I'm prolly gonna pull them out someday and make a couple slits in the carpet so that they are hidden a little better.
Now that all the wires were ran the rest is a breeze slide the two mounting bolts into place on the mounting kit and slide it in and put on the two retaining nuts, instructions are included as well. Then put the factory trim piece over it and install the center console. TA-DA.
Like I said before this was on
my 92 Eldorado w/ non-bose system. Yours maybe have the head unit located in a different location, but mounting hardware and aftermarket harness should all work.
I still have to hook up the radio as the plug for that goes into the head unit in the trunk as well, but most automotive stores, that have a car audio section carry, decent sized extensions of these cables. Hopefully I can get some time to get the digi-cam out and get some pics.
Just finished installing the new Alpine!!!(Warning:VERY LONG) 
