Bad headlight switch?
 

Truck is a 72 GM K15.

What's happening is with the headlights off my turn signals work fine but with the marker or headlights on the turn signals don't work. At best the signal indicators might have a bit of a dim glow but they do not flash.

The 4 way flashers work with the headlight switch on or off but they only flash at half speed with it on. Don't know if that's normal or not.

The wiring in the truck seems to be in good condition and it has not had a bunch of aftermarket garbage hacked into it.

Just wondering if you guys think I'm on the right track or if I should be looking at something else.