First check the fuses in the fuse box and there is a seperate inline fuse in the fuse box that needs to be checked. There is a second inline fuse that is for the radio. When the a/c fuse(s) blow the system is put into a safety mode. You really need to get your hands on the ACC manual. The system is not easy to work on without it.
I've recently retired my 450 and I'm selling all of my manuals and support material on EBay.
http://motors.search.ebay.com/_W0QQsassZjrb18
Good luck, the 450SL has very classic lines.