Trace the wiring to a connector and check the voltage on the supply side. Should be >=13.5v with the engine running.
If ok then check the resistance through the grips although I don't know what they should be apart from the same as each other. Someone will be along soon to tell you.
It may be possible to run a temporary supply direct to the grips bypassing all the control gear but you'd need to check that the grips take the full system voltage on the high setting otherwise you may burn them out.
If both work correctly without the control circuitry it becomes a boring detection job . If both are equally poor then the switch would seem a likely suspect or possibly a relay. If the latter is suspected check which terminals do what and temporarily wire the relay out of circuit by bridging the appropriate connectors. You can often take the cover off a relay and use abit of fine wet and dry to clean up the contact faces as a temporary fix.
Don't sample the Monkey Juice until you've finished the job for the day
