no cruse or brake lights// w headlights on

i;m not sure when this started. cruse has never worked with head lights on since i bought it 3 years ago. now my 1997 G 1500 converstion i figured out doesn't have brake lights or cruse cont. with head lights on. I have no other elect. problems and never have/ I did install an inline towing harness when I first got the van but can't ever remember the cruse working with the lights on