Where exactly unders the dash is the brake light switch located on a 2003 honda accord

Next to the brake pedal... up under the dash near the pivot.

It's located above the brake pedal arm. Look for the brake switch pedal stopper pad, which is often dark blue (replacement pads are white, Honda Part No. 46505-SA5-000). Screw in the brake switch until the plunger is fully depressed (threaded end touching the pad on the pedal arm). Then, back off the switch 1/4 turn to make .01 inch clearance between the threaded end and pedal topper pad. Tighten the locknut firmly and connect the brake switch connector. Pedal stopper pads often crack and fall out on the floor mat, causing the brake lights to stay on and the battery to run down.