[Kennuck]

Grounds can do some funny things for sure. However in my case the headlight switch was a real issue confirmed by bench testing with an ohm meter. After I received my new switch I disassembled the defective one and found the contacts bent and out of place causing the problem, likely occurred during assembly at the factory.
Check your headlight switch with an ohm meter for continuity between the main power supply and the power out to your park lights it should not have continuity when in the off position.