got a 1999 Mercury Grand Marquis. the car runs fine until you hit the brake or turn on the headlights, which at that point it stalls and dies. what do I do?

I take it that there's no "Check Engine" light on? Remove the intake snorkel and clean the Throttle Body plate with Carburetor cleaner, a soft cloth and your finger. If you're careful, you can remove the Throttle Body for cleaning. NO ABRASIVES! You can also remove the two small bolts holding the IAC in place and clean the inside of it as well. Try to move the plunger inside of it while bathing it in carb spray also. Disconnect the battery to reset the computer. Assemble the vehicle, start and run the engine at 2500 RPM for a full minute or two (warm the engine) and allow the car to idle for 2 minutes. Set the brake and allow the car to idle in gear for 2 minutes. With the car in gear and at idle with the brake set, turn on the headlights and AC and let it run for 2 minutes. It's typical for the Throttle Body and IAC to become sludged over time and require cleaning.