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I recently purchased a 79 350 and it had a motor swap and also had a front end from a 77 installed on it at some time before I bought it. I've been making a check list to make it road worthy since it sat for about three years. One of the issues is the front end lighting. I know I have misc bulbs to replace but an issue is the front turn signal/parking lights. The parking lights do not come on but the front signals work. are those bulbs dual filament bulbs that the parking lights are burnt but the signals work? Also the left signals work fine but the right blinks really really slow. Fuses maybe?
Front end wiring is put through two square like plugs on both sides. Look for them near the frame horns, should be easy enough to find, follow the wiring to the front.
Use a trouble light or multimeter on this plug to see if voltage is heading to it. And then you can use this point to check for continuity to the bulb sockets. The lights ground to the radiator support.6
What ever you do, don't cut or poke the original wiring! It's old and adding holes into the wire insulation just allows another way for water to get in.
Look up the wiring schematics (Ford Service Manual shows all the wire colours) to help sort out which wire is which, but using a test light one can find the purpose of each wire.
Personally, I really don't like the original sockets dentsides uses and have 'upgraded' to newer bullnose (and newer ford) type sockets and they fit the old holes.