check that the ground wire(s) are good and that they have a good frame ground.
some ground to the radiator support frame
make sure this has a good ground to the frame
take a volt meter VOM and check voltage @ each light prong. if that is lower than niminum 11.5+ volts you also have a problem w/ wiring on the pos. side of your circuit. you can also check resistance to gnd w/ the VOM to gnd and @ the H/L ground wire. so check ALL connections. a good way to make sure all connections are good is a solder/ heat shrink joynt, simply crimping a solderless connector on the wire is so-so. clean your H/L plug 3 prong, foot switch prongs as well as the H/L dash switch terminals so no surface corrosion is present.
even better yet is to use a relay for each H/L~ hi-lo circuit
http://www.madelectrical.com/electricaltech/brighter-headlights.shtml http://www.madelectrical.com/electrical-tech.shtml http://alanhorvath.com/54chevy/mad_electrical.php &&
http://alanhorvath.com/54chevy/understanding_relays.php
