Ford Focus mk1 tail light not working when brakes pressed?

So the problem is that when lights are ON and the brake pedal is pressed, the right taillight goes off. I already replaced the bulb and cleaned bulb connector and ground connection on the body. The left taillight is working fine. The connectors seems fine on both taillights. What could be the problem?

There's a short somewhere, check wires especially the connectors and where they pass through bulkheads.

Get a multimeter or test light and check for power and ground at the connector with the pedal pressed. Could be a bad wire or connector or socket/terminal.

Bad ground is still favorite.

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