There have been a few discussions in the other areas about suitable replacement radios for our C3s. I found myself in the same situation with my '72 Chevelle. I needed a radio and did not want to start cutting the dash apart.
I called Crutchfield and explained to them that I wanted a radio and CD changer that could be mounted out of sight and would work from a remote. The salesman looked through their inventory and found me a nice Sony am/fm cd changer and a seperate 6 disc changer with wired remote. I would have prefered a wireless FM remote but I bought those three pieces plus two male to female antenna extenders and got a trunk installation kit delivered to my home for under $500.00. The remote is $149.00 by itself and will work with any Sony head unit or CD changer that is equiped to operate with a wired remote.
I'm sorry I did not know about this type of setup three years ago when I ordered that worthless USA5 unit from Custom Autosound for my Vette.
Anyway, I just thought I would pass along the fact that there are inexpensive units out there that do not require even removing your original radio, let alone cutting the gauge bezel.