Route the two warning light wires back from the control box, under the carpets, to the warning light. Join the two wires to the two terminals on the warning light. Using a Scotchlok connector, tap a further wire into the red warning-light-to-control-box wire, then join this to a fused, ignition-controlled feed.
Brake lights are a vital warning signal to other road-users and a legal requirement, so you need...