You did the right thing using two relays and 10-ga wire. 12-ga wire will only handle 20 amps over 15 feet without starting to have a voltage drop. Even 1/2 a volt will cause significant loss of light output! It's not really a fire hazard until you get up to 30-40 amps
For anyone interested I sell basic reverse light kits a lot cheaper than $58.00! They use standard automotive style 30 amp relays (40 available for higher current lights) or I can custom build to any wattage you can come up with!
I use 12-ga wire for lights up to 110 watts total and 10-ga wire for anything bigger.
Everything is included except the lights. These kits are custom built for the Xterra! Full auto kits are also available. These allow you to switch the light(s) on manually, have them come on automatically anytime the truck is in reverse or never come on at all.
See
My Pricing Page for details of how the kits function. There are other kits now available also!
Also
My Web Page shows more pics of these kits.