2002 Silverado: Brake Lights Only Work When Headlights Are Turned On

I have a 2002 Silverado LS, and I have never really had any problems out of it.  About two days ago I got pulled over because my brake lights wern't working.  Anyway, after tinkering around trying to figure out what was wrong, I realized that everything works perfectly only when the headlights are on.  The truck has automatic headlights and daytime running lights.  Well, the brake lights only work if I manually turn the switch to turn on the headlights.  I also noticed that the daytime running lights aren't coming on at all!  I really think it has to be a switch, fuse, relay, etc. (at least I hope it is), but I just don't know.  COULD SOMEONE PLEASE HELP ME?!?!  The Chevy dealer told me it would cost $110 just to look at it!  I just don't want to pay these guys $110 to replace a fuse.  THANKYOU!!!  

I forwarded your Question to Alex Steal, Truck Trends man on Truck Trend Garage. Hopefully he can help.  

Late model exterior lighting circuits are not as simple as you might think, and your brake lamp problem can have a number of potential causes. But you're right, start with the basics. Check all the fuses (under hood and under dash), especially the 20 amp "Stop LPS" fuse located in the under hood fuse block. The 12 volts for the brake lights originates at that fuse (assuming there is voltage at the fuse block feeding the fuse). From there the power goes to the brake light switch at the brake pedal, and then past a few other connections before actually reaching the brake light bulbs. The brake lights should work even with the headlamp switch and the ignition off (hot at all times). Just a guess, but it sounds like that fuse may be burned out and you're getting feedback voltage from another circuit when you turn on the headlamps. Of course, the cause of an overloaded circuit which burned the fuse is another ball of wax. If all fuses are good, and power is applied to the brake lamp switch with the ignition and headlamps off, it gets more complicated. If that's the case, it may be worth the diagnostic charge at Chevy service to save you the aggravation.