Odd-Jarle,
when you bought the car in Germany, did you get any papers from the seller, such as the Kraftfahrzeubrief and TUV certificates? If so, then you have the name of the area hwere the car was last registed. You might contact these Zulassungstellen, if they can help with old documents, here are all German Zulassungsstellen listed with complete contact addresses
Das KFZ Portal - KFZ-Zulassungsstellen