1990 celica dash lights

I'm having a problem with my dash lights. They turn off everytime I hit a bump and turn on again next time I hit a bump. Sometime they don't turn on at all unless I tap tap the dimmer for a minute or two. It's probab;y a loose wire, but I don't have the slightest idea where to start looking. I would appreciate any advice. I'm trying to learn but I don't know much about cars so please keep it as simple as possible. Thanks for your time.