There are some people that advertize amp repair
and radio repair.
Repair resources
http://www.bmwgm5.com/Logic7_6941681.htm
http://cgi.ebay.com/ebaymotors/BMW-C...ioQ5fVideo#ht_
http://www.bmwradiorepair.com/?gclid...FccbQgodIEn0Cg
If you have the Hi Fi amp(premium system) you might as well replace it with
a Jl audio 600/6 and a technic harness . L7 is much harder.