I added some extra circuits to mine so I wouldn't wind up cutting into the original harness. I didn't want to take any chances over-stressing the old wiring with lots of new electrical accessories: power windows (all 4 - quarter windows, too), power locks (and a trunk popper), electric fuel pump, back-up camera, modern stereo equipment, alarm system, fuel injection, and a set of driving lights. Hey - it's a restomod, after all.
A straight tap from the battery, 30AMP circuit breaker, into a relay where the hot side goes to a 6-circuit fuse block (constant power), and the relay side to another 6-circuit fuse block, with the relay trip coming from a keyed power source. It's all protected by a Plano bait box.
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It's not the prettiest, but it works like a champ. At the time, Painless Wiring wanted over $300 for a 6-circuit kit... I'm into this a whole $70 with 12 circuits.
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I still haven't wrapped it in carbon fiber vinyl yet, and I think I might relocate it to the apron under the starter solenoid, trading places with the voltage regulator so it isn't quite as noticeable.