Other possibilities:
1-Fuses on these cars can degrade over time and appear to be good but actually be going intermittent. Replacing the fuses can fix a number of weird electric gremlins. Ideally, try to avoid aluminum ones as they corrode more over time.

2-The circuit board in the control unit can develop cold-soldered joints as it ages; do a search for how to take it out and re-solder the connections on the board, which is often effective and took me about a half-hour.

__________________
Craig Bethune

'97 SL500, 40th anniversary edition

'04 Olds Bravada (SWMBO's)
'06 Lexus ES330
'89 560SL (sold)


SL--Anything else is just a Mercedes.
(Kudos to whoever said it first)