I went through this issue last night.
I've been noticing that my grips were not as hot on high as they were on low so I attacked with a multimeter last night and believe I found the problem.
I checked the resistance of the switch through both the high and low terminals to center, and found that the resistance was .5 ohm through the low terminal and 1.8-2.5 ohms through the high terminal. Obviously the switch was moved to the appropriate position for the test. I then sprayed some WD40 in the switch by dripping it adjacent to the rocker itself, then flipped it back and forth about 50 times to scrub the contacts inside. The resistance afterwards was .5 ohm on both high and low settings. I then tested the amperage draw through both high and low by reconnecting both the high and low side terminals but placing the multimeter between the hot wire and the center terminal. I noted about 1.5 amps on low and 2.8 amps on high (from memory) so I think I've found the problem.
I guess these switches need to be protected from the weather.
I have not tested the grips on high in a real world test yet.
I don't know whether the WD40 is partially responsible for correcting the resistance readings or if it was just cycling the switch. I don't want to take the switch apart to actually find out whether there is any weather protection built in as they are often impossible to re-assemble.
The lesson here is either get a weatherproof switch or place it somewhere out of the weather.
BC