electrical problem on a 77 f250

i have spent countless hours trying to figure this one out and i am still unsure what is wrong. when i turn my lights on they pulsate. bright then dim, bright then dim. i have put a new battery, alternator, belt, voltage regulator, solenoid, checked fuses and ground and i still can't find out what the problem is. It gets worse when i give it gas, its just contantly bright then dim, but it doesn't do it AS much when its just idleing. any thoughts on what this could be?

Lose wiring to the light assembly?