Door opens-power seat stops working

I have a 1991 300sl (r129).

The power seats wont work unless either door is open. Its really weird. At first I thought that a fuse was out because the power rear view mirrors also didnt work. But when I open either door, the seats and rear view mirrors will then operate. I have no clue where to start.

This website has been really helpful to me. I am very good at fixing stuff but not at diagnostics.

Please help