Working on a 72 Mach 1.
Directional lights work until I turn on headlights or parking lights. Then they stop functioning.

Thanks to all that can help.

 

Don't know how much help this might be,
The one thing that is common both when head and pkg lites are on is the tail lites.
So,, start looking there for a poor or absent ground for the tail lights.
Sometimes as a shortcut , I will run a separate ground piece of wire from a good ground location to the bulb ground at the tail light(s). Remove a bulb plug or lens to do this. All you need to do is touch that grounded wire to the base of the bulb when the signal light for that side is on and the parking lites are on, if the ground is the problem everything will work like it should until you remove the temporary ground. Let us know if addl suggestions may be required.