fuel gauge problems

[jnbking]

I have a 78 and changed the gauge cluster to 82 model out of a parts truck of mine. The only problem that I'm having is that when I turn on my parking lights or my head lights my fuel gauges goes to empty. It seems to work just fine as long as I dont use my lights. Any sugestions would be very helpful. I can turn the truck off and back on and it works just fine just can not turn on any lights. Thanks