IF they are of similar vintage (e.g. R65 and R100 both either 79/80 models, or both 81-84 models) you have the best chance. The wiring harnesses are still a bit different, color code-wise, so you'd have to pay attention when looking at a Haynes manual or consulting with other R65ers for wiring harness info. But at least the similar vintage units have the similar types of switches, fuse block, relays, etc. so should be compatible, if not identical. If the bikes are of different vintage, it becomes much more of a "custom" deal, as color codes, grounds, switches, etc changed over the years.