You could try to contact the KBA (Kraftfahrtbundesamt = central registration office) and see if they are willing to give you any information on the previous owners using the VIN number of your car. Privacy laws might be a problem here, but probably they can at least tell you how many previous owners exist.
Contact information:
Kraftfahrt-Bundesamt, Fördestraße 16, 24944 Flensburg, Germany
Phone: +49 (4 61) 3 16-0, Fax: +49 (4 61) 3 16-14 95, web page:
www.kba.de
Good luck,