What do sitcom car rigs look like without effects?

Typically seen in shows with a live audience, there's a very noticeable type of shot where the characters are in a car with what's presumably a blue screen background, the car is obviously stationary and camera movement and usually slight rocking of the vehicle are used to give the impression of movement, for example from Frasier:

Obviously there's going to be a monochrome screen behind them to composite the shot onto and some kind of moving lighting rig to emulate streetlights, but what about under the car, is there a special 'car rocking' rig at the studio, perhaps some intern out of shot, or even a fake car?

The car seems to be real in Frasier though, and the closest thing I've seen in the past is a recurring joke from Garth Marenghi's Darkplace which has a very fake car rocking about in a small darkish room:

Which makes me think the rocking rig isn't as implausible as I had originally considered, as the very centre of the car doesn't move.

Pre-empting comments, please note I'm only interested in the sitcom style as the audience being behind the camera precludes the movie style effects detailed in this question.