1997 chevy Silverado engine stalling

I have a 1997 Silverado starts just fine , idles just fine , as soon as you step on the brake to put it in gear it stalls , checked vacuum leaks , none. so while it was idling I was playing with the switchs , I noticed when you turn the head lights on it would stall, stepped on the brake with out trying to put it gear , and it stalled. removes the ABS relay it continued to idle, but had no power brakes. replaced the ABS relay. it stalled again when stepped on the brake. so it appears anything to with lights makes it stall. all fuses are good, all relays are good, wiring looks fine no burns or anything. in fact everything functions as it should , until you step on the brake or turn on the headlights. headlight switch is also new. don't know if this is relavent , but courtesy lights inside the cab has never worked . cluster lights works as they should.
anyone know what the problem , I would be grateful for the help on fixing this, Thanks in advance.

Have you looked at your egr valve to see if it is opening when the engine starts up?

the lights don't use very much power but they can cause the alternator to draw more power and reduce the idle speed. The IAC is supposed to raise the idle slightly to keep the engine running. Use a scanner and see what the IAC is doing and what the fuel trims are.

If you turn up the idle speed just a hair, does it stay idling then?

Your brake booster is going out. Eventually it will get so bad when you step on the brake the engine will die.