CC,
If I'm not mistaken, wouldn't your L+ to the switch originate from the coil output "Y"? Your electric to the taillights shouldn't be affected. Your DPDT switch should alternate between your headlight and your grip heaters. Center position should still be off, while the switch for the grip heaters should be resistor controlled for the low heat setting and a lower resistance value for the high heat setting. If I had any clue to uploading diagrams, I would. Basically, by your drawing, your shunt should be connected to the coil output "Y", and the coil output "Y" should be a constant connection to the taillight, unless, of course, you want to switch the taillight at the same time as the headlight. Then wire the taillight and headlight to the same pole on the switch you have controlling the lights. Hope that helps