All true. Also I'm trying to think of a reason why anyone would buy a Buick Electra in Germany for export, which was the usual function of Zollkennzeichen; I've only ever seen them on German cars. I'm not saying it's impossible or even improbable, but it might open a discussion on the use of Zollkennzeichen.
If the season and episode numbers match up to the ones on the IMDB, this episode is from 1987, clearly making the Electra a secondhand car. I may be wrong, but I'm fairly sure that Zollkennzeichen are used for secondhand exports.
Yes it's from 1987. In movie the car was rented (or something like that) in West Germany - it's not visible - and then arrived to Poland.
Movie makers probably didn't even care about what plates it should has, they installed what they had just to give a foreign appearance to the car.
You're quite right; way back when, when students, hippies (and plastic hippies) from the USA toured Europe in elderly type 2 VWs and the like, they invariably had Zollkennzeichen - which stayed on when they sold them in London or Amsterdam before going home.