The differnt between wiper motor, built in from year 1988 and later to the wiper motor up to 1987 is the form of motor shaft.
To see on the picture below is the conical shaft of a wiper motor built-in from year 1988. In contrast to this wiper motor the wiper motor built-in up to year 1987 has a cylindrical shaft.
Conical shaft of windscreen wiper motor, buit-in from 1988:
