One part that's notorious for getting a high resistance connection is the dimmer switch. It's down there in a very unfriendly place, and it gets lots of road splash and salt, etc. Try running a temporary wire directly from the battery to a headlight, and see if the light brightens up that way. If it does, backtrack one connection at a time until you find the bad connection. It doesn't take much resistance to create a marginal connection on a 6V system.

The old school fix for dim lights was a headlight relay that took most of the load off the headlight switch and dimmer switch. They're still available, I think.
Jerry


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