Finally got the gps/log/echo -problem solved and seatalk working again.
I phoned Sealine and they were extremely helpful. They phoned me back in a couple of hours, after a talk with the engineer who does the wiring on the F37!
With their instructions I managed to track down the faulty connector in the furthest corner of the boat.
The connector fed 12V to the +/- wires of the seatalk. It had got moisture and was totally gone. Replaced it with a new connector - problem solved.
Big thanks to forum members for helping me out and to Sealine for a very good service.