The best wiring harness is the one you make yourself. Every project is different - I just can't see how a store bought harness would fit everyone's purposes.
On our truck we only have one relay - the horn relay. That's all it needs. (OK, I suppose there's a relay in the regulator).
On our car project, I used relays for the headlight door limit switches, electric fan, horn and driving lights. (i.e switching high current loads)
I don't think anyone should "fix" thrashed wiring and worn out switches with relays.
