Where is the relay that control brake lights on a 1994 Oldsmobile CS Coupe?

There isn't a dedicated brake light relay in a 1994 Oldsmobile Cutlass Supreme Coupe (assuming that's what you mean by CS Coupe) in the same way some cars have a single relay for all brake lights. The brake light circuit is typically controlled directly by the brake switch and the Body Control Module (BCM) or a similar integrated control unit. There might be relays involved in the overall power distribution to the lighting circuits, but not specifically *just* for the brake lights.

To find the potential source of a brake light problem, you should:

1. Check the Brake Light Switch: This is the most common cause of brake light failure. It's located on the brake pedal assembly and is relatively easy to access and test. A simple multimeter can check if it's closing the circuit when the brake pedal is depressed.

2. Inspect the Wiring: Look for broken, corroded, or loose wires running from the brake light switch to the taillights. Pay particular attention to areas where wires might rub against metal parts.

3. Examine the Bulbs: Make sure the brake light bulbs themselves are not burned out.

4. Check the Fuse: Consult your owner's manual to locate the fuse for the taillights/brake lights and check its integrity.

5. Body Control Module (BCM): While unlikely to be the primary cause of a simple brake light failure, a faulty BCM *could* affect the brake light circuit. This requires more advanced diagnostic tools and is best left to a mechanic.

In short, you're unlikely to find a separate brake light relay to replace. The problem almost certainly lies elsewhere in the circuit. Start with the simpler checks listed above before suspecting a more complex issue with the BCM.