The ground (cold side) of the brake light signal in a Sprinter can sometimes be found in the wiring above the brake pedal. A high-sensitivity LED circuit tester will be required to test this due to the weak signal strength in this wire. This is due to the low voltages in this vehicle's wiring system.
In other cases you may need to install an accessory stop light switch kit # RM-751475 to provide that brake light signal needed to trigger a brake controller like the Prodigy P3 # 90195. view full answer...