I've never looked into it, relays offer less resistance than the switches?
Well, a new, quality switch and relay should both have negligible resistance, but I think crusty old trucks are a different story and also tend to have undersized wiring and such. I bet you'd find a good bit of resistance from the battery through the rocker switch to the tailgate. Relays made a huge difference to my headlights, though I never specifically measured the resistance through the pull knob and wiring.
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