The correct wire on a vehicle to connect to the red input signal wire on a brake controller like the Prodigy P2 # 90885 is the one at the cold side of the brake light switch. The cold side of the switch is the wire that shows 12V power ONLY when the pedal is pressed and at no other time. Use of a voltmeter or circuit tester like # PTW2993 will make it easier to identify the correct connection point for the controllers input signal.
The linked video shows how the tester is used to detect... view full answer...
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