lets see, basically its a three position switch. you would be drawing twice the current which probably isn't a whole lot for lights. i guess to really answer this question we need to know the wattage of the bulbs and what amperage the contacts in the switch can handle and what the gauge of the wire is. that isn't a huge bundle of wires coming to that switch so there could be other things using the hot wire. with a wiring diagram i think i could figure this out but i don't know how we could find out what the switch contacts are good for. anybody know how to find this information?

 

buy yourself a good water proof relay, use the high beam for a signal, then use the brake light wire for power, hook through relay to the lower lights and ground to frame.

 

You would not be drawing twice the amperage just the added current of the high beam (approximately 3.5 amps) the rating of the switch is not all that relevant due to the fact
It's in a position where the contacts can be barely making contact. I would highly recommend using a relay to achieve the function. The 250 watt alternator will have no problem supplying this additional load