That's the central locking unit.
This is the thread and the fellow's handle is bstreep, not B. Greer (sorry, Bill) - I did it! Keyless entry for your 124 made easy (and less than $100)!
FWIW, I don't think Crutchfield sells the relay pack anymore. It's easy to make your own, though.
83 300SD ... $1100, see cars forum