This may help diagnose. Then let James do the soldering unless you have a time/inclination to mess up a unit while learning.

Middle-aged Mercedes Electrical Repairs

Scroll down for some diagnostics.

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85SD 240K & stopped counting painted, putting bac together. 84SD 180,000. sold to a neighbor and member here but I forget his handle. The 84 is much improved from when I had it. 85TD beginning to repair to DD status. Lots of stuff to do.