troubleshooting those is a fairly straightforward procedure.
first thing i check is continuity on the grips. unplug them from the main harness and check each grip independently. no continuity, no heat. dont recall which grips were on those but it seems like those had the grips where the wires used a 90degree spade type female connector on the wire where it meets the grip with basically a large rubber band holding them in place. if that sounds like yours, they corrode or come unhooked. check the connection as well as where the wire goes in the female connector. they break there often.
if you dont have means for testing continuity, hook a 12v automotive type battery charger to the bullet connectors where you unhook them from the main harness. if the grips are good, they will heat up.
from there its a check of the magneto's output after chasing down the yellow wire with the black stripe and checking the ground where it plugs in. everything else work like dash lights, etc?