Q: I have a 1987 chevy blazer S10, replaced brake light switch still no brake lights, whats wrong?

There is either a wiring break or perhaps excessive voltage drop on the circuit, that is a high enough resistance to prevent the required voltage from reaching the bulbs. Whatever the cause of the circuit failure, a Mechanic would simply trace the relevant circuits, using the wiring diagram in the Factory Service Manual, find the wiring, ground, termination or device fault that has caused an interruption in power and repair it. If you want these steps performed by a certified Mechanic, dispatched by YourMechanic right to your location, please request an electrical circuit/component diagnostic and the responding certified mechanic will get this resolved for you promptly. If you have further questions or concerns, do not hesitate to re-contact YourMechanic as we are always here to help you.