That's the central locking unit.

This is the thread and the fellow's handle is bstreep, not B. Greer (sorry, Bill) - http://www.peachparts.com/shopforum/showthread.php?threadid=11777&highlight=300e+remote+keyless

FWIW, I don't think Crutchfield sells the relay pack anymore. It's easy to make your own, though.

Sixto
95 S420
87 300SDL
83 300SD ... $1100, see cars forum