It's a relay behind the battery - you need to remove the plastic shield just behind the battery to get to it. It has a fuse on top, and I'm not sure of it's function, but it does tend to react to car washes. Keep a spare one handy just in case.

Vivian

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1939 170 - till I was 3, still turns heads
1989 300TE - daily driver has 263k
1987 300E - purrs happily with 120k
1983 300SD - keeps my friend happy