but I just crawled under the dash and I don't have any of the part numbers you listed. I guess the previous owner bought the wrong one.

Here's what I have.
1. 003 545 20 05 - fuel pump relay (I guess this is the wrong one for my '88 560sl)

2. 006 545 85 32 - Idle Control

3. 002 545 18 05 - a/c relay

and 2 of these little silver boxes:
001 542 02 19

Question: could the wrong fuel pump relay be the cause of my problem? Does that number go to a different car?