Probably a bad ground on the housing/light bucket.
To test, make a jumper from alligator clips. Clip one firmly to the housing, other to a good known ground.
I've done a Painless on a CJ. It's good, also expensive.
If I was doing one today I'd use an EZWiring 12-circuit kit for WAY less than a Painless. That's what many of the guys use over on EarlyCJ5.net/forums.