[dscott]

I'm not sure how to answer this one. To get a voltage drop something has to act like a resistor or the wire is too small for the current draw. I think I would start with the battery and have it checked. A battery can have a memory and only charge up to a certain voltage . I would also check the alternator size. If it is rated below 75 amps that will cause a problem when running several things such as lights, radio, ignition, etc. Usually when the lights dim it is a voltage problem and alternator sizing is important to maintain the proper voltage level. I have a car in the shop now that was experiencing similar problems when the guy runs his big wattage stereo and subwoofer system. We had to add an extra battery with an isolator for the stereo system so we didn't draw down the main battery for the lights, ignition, etc.
Don