I went through this whole process last October. The car I purchased had the dash redone, so the VIN plate was not attached. And for some reason, the VIN plate was removed off the cowl (My guess is for the repaint job.)
I went to the DMV for the inspection and they threw a fit with the VIN plates removed. I did turn up the dash plate in the glove box and that was quickly confiscated. I later found out that it is illegal to possess a VIN plate that is NOT attached to a car. I also found out that it is punishable up to prison time. (DMV has no sense of humor either.)
I had all the paperwork from the PO for everything he had ever done to the vehicle. I had a notarized affidavit as to why the VIN plate was removed and in the glove box. I even had another affidavit for the fact that the car had not been titled in the PO's new state for 4 years (PO was a college student during restore, and took a job out of state.)
I found a lawyer from a local classic car shop. (If you need a name, let me know.) The lawyer filed the petition to have the original VIN put back on the car, since I had all the supporting documents and the original title. The DMV wanted it to have new numbers. Cost was $500.00 for a quiet title. There were two weeks for the newspaper ad, then the time for the court systems. All together, the time was approximately 2 months.
During that time, I was able to get some repairs done on the car to make it more road worthy.
I really had fun going back to the DMV with the court order stating the original VIN was to be applied to the car. It even said, "Make it so." (sounded like Star Trek.) DMV squrimed all the while they installed the plate.
There are only three VIN locations on Opels that amount to much; the door, the dash, and the engine cowling. The number next to the radiator is the chassis part number and does not relate to the VIN, neither does the engine number. The door may have been replaced at some time, so it may be gone.
All that said, I'd be careful. I had thought of going the route of even buying a junk GT to swap the plates.
Good Luck.
(B.T.W. I found the cowling plate under the gas tank when I removed it for rust treatment. I'll be darned if I go back to the DMV with that one.)