One thing (And I see it suggested above so I will second and shorten my post)
IT can be a long long way from the battery to the light switch and back to lamp And that means VOLTAGE DROP
So it may help to put a relay near the headlights (or 2) and run heavy wire from a proper fused spot (Or use an inline breaker) to the relay to the headlight and to ground.. You need two relays for each side (High/Low) one breaker can feed both sides or one breaker per side (There is a good reason for using 2 breakers)
The existing leads power the relays.... NOTE this may mess up some DRL systems however.. like the GM system used on my Workhorse.