No power in cab

this F100 old school carby and coil and distributor

arc across starter motor nuts to see if it cranks over

start crank >battery > starter soleniod > ign key

spark = battery> igintion > coil >
distributor rotor @ number one cylinder on compression
leads to plugs

coil lead giving off strong arc at crank ?
spark at plug leads ?

Pretty sure there is no relay for the headlights. There are two 10 amp fuses in the fuse block, one for each side headlight. The power to the fuses is directly off the black fusible link (in your fuse link box, under hood, by the battery). Then the fused wires go into the headlight switch, and with switch on, out to the head lamps.

First check if the fuses have power to them. Hot at all times, so key can be off. If they have power, fuse link is good, so possibly a bad headlight switch or dimmer switch if no headlights..

after all fuses and relays verified are working it sounds like you have keyless entry on the door panel with a keypad and in either case theres a driver door module inside the door panel this may have gone or the fuse for that has blown I'm ssuming that you have the owners manual with the diagrams if not ford owners .com register then download your manual then go to roadside emergencies and the fuse and power distribution box diagrams are there also this could be you headlight switch so if the fuses are all good then lookat the door module on the drivers side next to the speaker or the headlight switch do the windows work also check the master window switch this is because all components have some borrowed power from other wire circuits so look carefully at all of these even a corroded wire or contact can cause havoc on other systems clean al and a light dab or spray of wd40 will help you good luck Stephen

Did the battery explode, or leak, do you have power anywhere else? If so you need a new battery. And yes there are other fuses under the hood, normally next to the inside of the fenders.

Sounds like a ground wire problem. Does it act up in the rain or after a wash? That could indicate the fuse block under the hood my be geting moisture. But definitely check the main ground, fuse Block ground, and headlights ground. Check headlights plugs n turn signal plugs n sockets. Also, start by changing the bulbs. Both of them. There's two filaments in each bulb, one for marker lights and one for blinking

if fuse under hood has blown there will be one in main fuse box that has blown as well fuses can look good but are bad need a comunity tester or multy meter to check connection

Could be Headlight relay, bad ground, bulbs blown, fuse blown or unplugged connector under the hood. The interior headlight switch is a long shot. Make sure everything under the hood is hooked up. Look around the firewall or headlights for unplugged connectors.

what fuses were blown and replaced? Theres a TBC fuse under the hood, along with a Headlamp Relay. Theres a R HDLP and LT HDLP fuses, make sure they are getting power, and are good. If the LT HDLP and RT HDLP fuses are good AND getting power, then the TBC fuse and the relay is good. then I would suspect the Hi-Lo dimmer switch bad. The LOW beam ground wire is YELLOW, the high beam ground is PURPLE. see if high beams work, if they do, then I suspect the dimmer switch is bad.

Most cars have a Headlight relay and within the relay different terminals will split the power to parking lights, headlights, etc. It is possible that part of the relay burned out while the balance still works. You may have a fusible link (wire) to the relay box for just the headlights. A good Autoparts place should show the requirements.

I suspect the bad connection on the battery caused the Starter to draw power or a ground through the easiest path of least resistance. This shorted the grounded Relay.

they are powered together so check the fuses, some will be under the hood and some inside the cab, the covers will have a list of fuses and amperage.