As has been suggested your best bet is to wire in a relay which is then connected to a small fuse box. I have done this on my Tenere and use the rear light circuit to activate the realy this then powers a fuse box with four outlets for heated jacket, heated grips and a cigarette lighter outlet which I use for the GPS. This leaves me with a spare outlet which I can use for my compressor when necessary.
I regularly use the heated jacket, grips and GPS with no charging problems whatsoever and for what it's worth I have HID's in both dipped and main beams.

Hope this helps.

Zou on this forum had a really good picture of this very same mod but for the life of me I cannot find it, perhaps he may come along and post it in this thread.

__________________
If I'm not working, I'll be