Q: license plate lights wont work on 1995 Ford Explorer

Rookie cbe0621eac06868b3efe0d8d1d3611e23c60d3114864ea2ec19a68cfbd3eebab
ive replaced the bulbs but nothing worked. i read online that it may be a bad ground connection between the socket and bulb so im gonna clean it and coat with some vaseline. if that doesnt work any suggestions?
(1) Answer
Best thing to do is to get a hold of the electrical diagrams and start testing.
here is a good source for those: