[dsraven]

check grounds in the bulb sockets and wiring at the trailer plug, if applicable. if one bulb has a poor or no ground at the pigtail or bulb socket then the power goes through the bulb element and the bulb body (where it would normally ground for either circuit) then, because it has a poor or no ground circuit, power feeds back through the other element sharing the same ground point and then exits in a way that is back feeding power (although less voltage due to the resistor effect of the bulb elements) through the other circuit of that bulb socket. in this case the low beam. you can easily check for a bad ground by simply turning on the lights and checking for one that is dim. that would indicate a poor ground in that socket or pigtail and the bulb is searching for a ground but can only find a circuit through another circuit. . if you find a light like that just check that socket for corrosion or pigtail eyelet ground point to ensure you have a good ground. I have often used a jumper wire from the pigtail ground wire to a known good ground point and if the bulb lights up like it should then the ground for that circuit needs attention. if the headlights are grounded to the rad support and the wiring there appears to be in excellent condition, like a good solid ground with a star washer under the eyelet terminal and no corrosion, then you may need to install a ground wire from the frame to the rad support because the rad support is relying on metal to metal contact with the rest of the body parts to complete a ground circuit. also, check to ensure you have a ground wire from the battery to the engine-like battery cable size, and one from the battery to the frame-like battery cable sized, and another from the battery to the body-like big enough to run everything in the accessory circuits-lights, stereo, airbag compressor etc etc.
make sense?
I have found that if you turn the lights on and a signal indicator comes on dim then there is a ground issue in one of the lighting circuits. start pulling bulbs till the turn indicator light goes off-bingo- found the socket that is the troublemaker.