A while ago (year or so) i had the heater core in the car changed. Ever since, the buttons associated with the AC on the dash are no longer lighted. The other night, I noticed they happened to be working, but as soon i pressed one of the buttons, the lights in the buttons went off again. Anybody know what it could be? I thought the light bulbs inside the unit all burned out until i saw the lights on the other night. The plugs behind the unit are secure, i've checked. Thanks in advance.....