The front ones from any year (88-2000) will fit. But the back disc for the 88-89 models are different from later ones. The EBC rear disc for the 88-89 is MD1127, for 90-2000 years you will need a MD1077. Pads are the same for all Gl1500s.
I have read somewhere that the 90-2000 rear disc will fit on the 88-89 but that the caliper mounting bracket must be changed on the 88-89 to the same as the 90-2000, and that the 88-89 caliper itself is OK to use on the 90-2000 disc.
Is this true, or have been lead down the garden path?????