Before you guess, I would strongly urge you to find the connectors for the left and right heated grip elements. They are two wire plugs. Same style for both sides. Mine are gray plastic. Once you find them, unplug them, just for good measure, cycle the ignition, then test each grip element by using a Digital Multi Meter and touching the positive probe to one pin, the negative probe to the other pin, both on the element side. If the grip element has integrity, it should show continuity between the pins. If the element is bad, there won't be a connection between the two sides of the grip element.
If the elements appear to be ok, good continuity between the pins, plug them back in and see what happens when you turn the bike back on. If one or both grips shows bad, that may be your answer. If the plugs have been cut off, or wires spliced in to the plugs, that may suggest they are not Yamaha grip heaters and why they don't work thru the ECU.