Inspect the wires that connect to the dimmer switch for looseness, brittle insulation, corrosion, etc. It appears that someone has misrouted the power wire tp your light switch- - - it should be connected directly to the ammeter, and the built-in circuit breaker in the light switch protects against shorts downstream of the light switch. In case there is an overload, the breaker is supposed to cycle on and off fairly quickly to avoid losing the headlights entirely. If the truck has had any kind of rewiring done, during an older restoration, maybe, it sounds like someone who didn't understand that principle wired the headlights through a fuse that also feeds other circuits. I'd suggest for starters that you wire a circuit directly to the light switch with a 30 amp circuit breaker in the supply line, not a fuse. That will take some load off the fuse that keeps blowing and also give you a safer way to protect the headlight wiring.
Jerry
