You can put in relays, but in my opinion, while that works, it is a bandaid which ignores the root cause of week headlights.
The ground side of the electrical circuit is the problem. It gets worse over time due to rusty sheet metal.
On a stock truck, the ground starts at the battery negative post, then to the frame. From there it has to pass from the frame through the front crossmember, through the radiator support, through the inner fender where the ground screw for the headlights go.
See the problem? That's a lot of rusty intersection to cross through.
I would sooner run a dedicated ground wire from the battery to those headlight grounds on the inner fenders, than to use a relay.
It's worth a try regardless. If it doesn't work you can use that wire to plumb your relays.
