Classic/Antique Car Repair: 1972 ford f100, digital voltmeter, gage wire

Question
Hi my blinker, brake lights, fuel gauge and speedometer are not working in 72 ford f100. we have replaced all of the fuses and are chasing down the wires, do you know how we could fix this?

Answer
This sounds like a series of problems not just one effecting all of the items mentioned. The speedometer is a mechanical device that is driven by a cable from the transmission. It attaches to the back of the speedometer. To check it disconnect the cable, pull it down under the dash so that you can see it when driving. take the car out on the road and while moving watch the cable to see if the inner shaft turns. If it does not then there is a problem with the cable or in the transmission with the gears that drive the cable. The fuel gage is not connected to the blinkers or brake lights. If the temperature gage is functioning then you know that you are getting power to the gage circuit as they both feed off the ignition switch. Disconnect the fuel gage wire at he fuel tank and with a digital voltmeter, not a test light, measure the voltage from the wire. It should be between 5 and 12 volts with the key on. If it is then the problem is with the sender in the fuel tank, if it is not then the problem is in the wire going to the dash or the dash gage itself. Now a question about the blinker and brake lights. When you turn the directional in either direction do any of the lights on the truck light up? That includes the front lights. They may not blink but it would be helpful to answer your question if you answer mine. Let me know.
Brad