My two cents:
When I purchased my 250SL, the car was pretty much a virgin, no abuse, no butchery, but the radio hole had been hogged out and a newer deck installed. Since the opening was enlarged and the deck was cheap, I bought a nice Panasonic unit with all the whistles because I too dig my tunes. But the volume sucked and I decided to install an amp. I can't remember the brand and model, but it fit PERFECTLY behind the glove box, on top of the passenger under knee guard card board panel. I simply rested it upon the foam padding, isolated all the wiring, and it works awesome. No one knows it's there, it misses the wiper arm as it rotates, totaly cool. And one more snipet, I was able to fit two 3" speakers on top of the same footwell panels without drilling holes and much improving the sound. Remember, no holes drilled, no damage to panels, can be converted back to stock, AND good tunes.
Maybe a help, or not, but it worked for me.
Greg