When you turn on high beams the voltage on the outside lamp will switch from the low beam filiment to the high-beam filiment (dual filiment bulb) and also energize the inside lamp. According to the wiring diagram the high beams are all tied together and fed with one wire.

I would suspect that you might have a bad ground on the l/h side headlight assembly. Or, maybe somebody had put in one of the heavy duty light harnesses with individual relays and maybe a relay is goofing up?

Good luck,

Justin