[powerdriver 1958]

That's not all that strange, Two common things to check.

Grounding,which this problem is. Should be a black wire that go's to the frame.They will windup broken or rusty at the frame


Some times a good place to start is trailer plugs,I have seen some pretty bad wiring done to hook one up.

Also this may help with wire color,

Left turns are yellow
right turns are dark green
tail lights are brown
backup lights are light green
Grounds are black