For BMWs, Mercedes, etc. (any car that has a chassis-code identifying its generation) it is not very important to have the model year. It is more important for cars that carry the same model name through several generations/models without having a chassis-code to regroup them.
So in this particular case, I think it is better to leave the year blank rather than putting 1988 for all, as many may be later models (restylings in the generation).