This is what I would do to troubleshoot your problem.
First I would see if I am getting voltage to the hot wire b/f the switch. Then I would test the voltage after the switch. check your ground, you can pull a continuity test to make sure that it is not broken.
If you are getting power to the grips, and you have a good ground, and your wire that ties the two grip heaters together is ok, then the only other option is that the element has broken, or burned out, so you would have to replace the grip heaters.
Hope this helps, good luck. There's nothing worse than not having your grip heaters when you really need them.
*UPDATE*
I made a stupid mistake in my prior advice. no big deal but I'd like to revise the procedure that I would take.
If you check the power after the switch, and you are not getting power then check b/f the switch. if you are then getting power b/f the switch, you know that it is the switch.
but if you are getting power after the switch, then you have no reason to check for voltage pre switch.