cruise control will not turn on when headlights are on

I'm having the strangest problem with my '90 240sx se fastback - when the headlights are on, I cannot turn on the cruise control. But if the cruse is already on and I turn the headlights on it still works.

Thanks for any ideas.


Update:
I realised that the cruise turns on fine all the time (i.e. the green light on the cruse button itself comes on) but if the headlights are on when I try to push the set button the green light on the dash will not light and the speed will not hold. If I turn on the headlights after the cruise is holding the car there are no problems.

anybody? please i'm so baffled by this...