Most grip heaters caution against modifying the wires in any way. You would be much better off to completely remove the old grip heaters and use the wiring that comes with your new ones.
FWIW, heated grips tend to work better than grip heaters. Grip heater by design are right up against the metal handle bars, which acts as a heat sink. Heated grips suspend the heating element inside the rubber of the grip, helping to insulate it from the metal bars.
If I was planning on riding in some REALLY cold weather, I would just skip the grip heaters/heated grips altogether and go straight to heated gloves. With heating elements covering your palms, fingers and the back of your hands, they would be the most comfortable. Some maker have finally started making heated gloves that are actually protective as well (Gerbings are about as protective as ski gloves). Of course, this does mean being "wired to" you bike, and they are now always on the bike, just in case. So heated gloves are not quite as convenient as heated grips.
Oh, and the chances are your rheostat is not custom. It's probably a custom mount with a Heat-Troller inside. That's what most folks use for heated gear if it doesn't come with its own controller.
Last edited by Warhammer; 08-09-2009 at 10:56 AM.