Warning, this subject is old (3093 days without answer)
DriveSmooth
The car appears in the opening scene of Episode 12 in Season 11. Aisha Tyler (as Dr. Lewis) drives up in the Opel GT and Joe Mantegna (as Agent Rossi) immediately comes over and tells her how much he admires the car. He then goes on to tell her about his '47 Buick, and there is a cool scene at the end of the show where they go for a drive together.
(After a little research, I found out that he's a longtime Opel GT fan and that it's highly likely the car is his in real life:
http://www.motortrend.com/news/cars-of-joe-mantegna/)
I found a YouTube video of that scene:
https://www.youtube.com/watch?v=i6lY1bbfgWo