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2000 Chevy express

I have a 2000 Chevy express van and brake light and cruise work until I turn on the headlights. Once headlights are on the cruise control and brake light stop working. Any ideas would be great, thank you.

All the circuits that are being effected pass thru the multi-function switch.

https://www.rockauto.com/en/moreinfo.php?pk=929397&cc=1358591&jsn=477

Common problem on GM vehicles.

Tester

Is there any way to diagnose that that is the issue? Or possibly the brake module.

The cruise control involvement points to a problem with the brake light circuit. Like when you press on the brake pedal the cruise control turns off, and it probably figures that out by the brake light circuit turning on.

From what I can tell the headlight power comes through a 40A under-hood fuse, then to the headlamp switch, then to the multi-function headlamp dimmer switch. It also t’s off to the daytime running light module (which has a parking brake function and a brake-psi function) and brake warning lamp function. The daytime running light module turns on a headlamp relay, which powers the actual headlights. I’d guess the problem is either the multi-function switch or the daytime running light module. Do the brake, high beam, daytime running light dash bulbs turn on when they should? And off when they should? Is the brake fluid level ok?

I will check that tomorrow and keep you posted. Thanks for the input.

Ok put new multi-function switch in and nothing. Also disconnected drl still nothing. But I did notice something, a noise from the BTSI ( brake transmission shift interlock) every time I would step on the brake. It would make a click noise. That is tied into the multi-function switch. Is the click normal or not? My other express doesn’t click like this one.