Welcome to the 'bolt, GM250. What part of Norway are you from? I spent a summer wandering the southern parts of Norway and Sweden; borrowed a buddy's car in Oslo and drove to the coast and back. Beautiful country full of really nice people. The beer is strong and the women are lovely; or was that the other way 'round?
I agree that a volt/ohm meter will be helpful in chasing leaking electrons. My wild guess is there's a bad ground somewhere. Just for giggles, why not check the continuity of the starter itself; if the windings are damaged you could be dumping voltage through the starter housing.
