How did fixing bad ground wire on running lights passenger side for 2000 Chevy S-10 affect brake lights. Now the drivers ONLY light goes off when apply brakes.All other seem work OK?

It is unlikely that fixing the ground wire on the running lights on the passenger side of your 2000 Chevy S-10 would directly cause the driver's side brake light to go off when you apply the brakes. Here are a couple of possible explanations for the issue you're experiencing:

1. Faulty Brake Light Switch: The brake light switch, which is typically located near the brake pedal, is responsible for completing the circuit that powers the brake lights when you press the brake pedal. If the switch is faulty or not properly adjusted, it may not be activating the circuit correctly, causing the driver's side brake light to go off.

2. Damaged Wiring or Connections: Double-check the wiring and connections related to the driver's side brake light. Ensure there are no loose, broken, or damaged wires, especially in the area where you fixed the ground wire for the running lights. A faulty connection or damaged wire could prevent the brake light from receiving power.

3. Grounding Issue: While you mentioned fixing a bad ground wire on the passenger side, there may be another grounding issue affecting the driver's side brake light. Check the grounding points for the brake lights and ensure they are clean and properly connected.

To troubleshoot the issue accurately, it would be best to use a multimeter to test the voltage and continuity at various points in the circuit, including the brake light switch, wiring, and ground connections. If you're not comfortable with electrical troubleshooting, I recommend seeking assistance from a qualified mechanic or auto electrician to diagnose and fix the problem.