Dredging up this old thread to add a reminder about voltage drop and wimpy factory wiring. As mentioned earlier in the thread, I put 796 bulbs into the new backup light fixtures of my 1986 F250, then I installed a pair of the oval LED backups Virgil recommended, into the rear bumper. Nice setup.
I often want a work light in back, or hooking up a trailer, unloading the truck, etc. I finally got around to installing a manual switch. I went brute force simple: From a stout fuse holder at the battery, I ran #10 (60 strand type SIS "hinge" wire, but that's the sort of scraps that turn up in my industry) under the cab and up to a push-pull switch with red indicator mounted on the gear shift, then on to the backup lights on the rear of the truck. BTW, in addition to chassis ground at the back, there's a #12 running ground going up front to the battery. I found that if I turn the truck on and hit reverse, I get backup lights, but switching on the manual switch makes a very significant improvement.
I haven't put a volt meter on the bulbs, but I'd guess more than a volt difference. You really see the lights get brighter with that manual switch. This is just a good reminder that factory wiring is usually barely adequate on its best day, so you need to beef things up a bit to get full performance from your bulbs.
BTW, that same truck has H4 headlights, standard wattage but premium Xtreme Vision bulbs. Adding a HD wiring harness and relays visibly improved lighting output there, too.