Why don't my blinkers work when my engine is running?

I have an 88 f150 and my blinkers won't work when my engine is running. They work when the key is turned but the engine is off but they won't work with my engine running. Why is that?

(it's not because it's a Ford either)

Wire shotage somewhere

Might be a bad relay or a blown fuse

It's very strange they would work with the ignition off and not work with the engine on. Study wiring diagrams, learn how the system works. Use logic to determine where shorts to power or ground could cause this.