The plug on the back might be loose, or....that plug is keyed, and the "key" broke on mine. I had to look up the wiring to get it right. I can definitely see it not working/blowing fuses if not installed using the (possibly broken) key.

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86 560SL
With homebrew first gear start!
85 380SL
Daily Driver Project

http://juliepalooza.8m.com/sl/mercedes.htm