Luis,
did you receive any documentation with your car? You should look for a document with the title "Fahrzeugbrief" or "Kraftfahrzeugbrief". You will find pictures of how this document looks like if you do a google search for "kfz-brief". The first entry in this document is the original license plate.
If you do not have this document, you still may have received a paper titled "Unbedenklichkeitsbescheinigung". To my knowledge, this is required for exporting a car from Germany. (You will have more experience with this than me!) This might also give the information you seek.
Third possible source of information: papers on the bi-annual road safety test ("TÜV-Bericht") should also show the registration number, but of course only the number which the car had on the date of the test.
Another option is to go through these guys, the federal car registry in Germany:
www.kba.de. I gave them a call just now regarding a question, and they told me that they will provide this information to the current owner (not other persons)
if the car is currently registered or has been registered in Germany within the last seven years. Apparently, they delete all data seven years after the registration has ended. I can assist you with translating, if required.
You could also try and talk to the people at the local registration office ("KfZ-Zulassungsstelle") of the city where the car was originally registered, but I have no clue whether they keep their files after a car is no longer registered within their area. I think this is rather not likely.
Good luck with your hunt, and let me know (via email) if I can be of help!
Aurel