Botched 300E system

[350SL4spd]

I am purchasing a 1991 300E from a friend that has a small audio problem. It's previous owner dropped in a very expensive mp3/cd player at someplace stupid like circuit city (not sure exact location). They rewired to all the original speakers, I believe bypassing the amp (not positive) and it sounded great. Really. For like two weeks. Then it started messing up. He brought it in a few times to where he had the work done and each time it would work for a little while then die. He got fed up and put the old unit back in and it worked for a few weeks until it crapped and now it only plays through the subs in the doors. I'd like to put the new head unit back in, but I'd like to know what I'm getting into. I searched this forum and couldn't really find anything on this type of issue, most posts were way ahead of me... Am I most likely looking at a blown amp? Blown speakers? Or just F-ed up wiring? Is the best option to get a harness from Crutchfield.com and start there w/ a fresh rewire? Will this even work or do I need to get new speaker no matter what? That's a good question: Do I have to get new speakers when installing a new head unit?!? I'd like to do this as cheap as possible as I just blew most of my wad on the car, but I also can not live w/o a stereo!!
Thanks in advance all!
-matt-