Many RV owners of Tow Vehicle Braking Systems prefer to have a Notification Light on the dash of their RV to warn them that the Brake in the Towed Vehicle is ‘ON”. This light is more likely to be seen while driving than a warning light visible in the Rear View Camera Monitor.
For this article we’ll reference the SMI Air Force One, a popular tow vehicle braking system used on coaches with air brakes. SMI provides a very bright LED warning light that can be draped over the Towed Vehicle’s rear view mirror and viewed in the Coach Rear Monitor, or the light can be attached to an arm of the tow bar with the light aimed up towards the rear camera. This approach is typical of many similar braking systems that use the towed vehicle’s brake switch to provide a signal that the brake in the towed vehicle has been applied.
The wiring required to provide the light power to the front of the coach is already in the coach from front to rear on most diesel chassis and on the post 1999 Ford chassis. Coaches assembled on these chassis are equipped with a seven (7) pin blade or "Trailer” connector. The wire referred to is connected to the "Brake” pin on the 7 blade connector. This wire would be used if a brake controller for a trailer was installed to provide brake power to a trailer’s electric brakes. In the example of towing a vehicle, the same wire is used except that power is provided from the Towed Vehicle’s brake switch or the braking system’s activation light signal power.
The tow car should be wired with a six pin round connector that is usually supplied by the cable extension maker. See this article to properly wire this connector for towing (https://www.rv-partsplus.com/properly-wiring-a-tow-vehicle) The Brake light wire from the towed vehicle is connected to the Brake pin in the six pin round connector on the front of the towed vehicle which will transfer the power to the coach. You will need to go to an automotive store, Walmart or Canadian Tire to obtain a 7 pin blade to 6 pin round adapter. Insert the blue cable extension from Blue Ox or the red cable extension from the Roadmaster cable into the back of the adapter and cross wire tie the "ears” on the Blue Ox/Roadmaster cable to the holes on the adapter. This will provide you with the Coach end and the remaining cable end will fit the connector mounted on the baseplate pins on the towed vehicle.
The Ford chassis typically uses a green insulation wire to either feed power from a brake controller to the Brake pin on the rear 7 blade connector to feed power from the rear connector to the dash area on the Coach. Spartan and Freightliner (and most other chassis makers) provide a "drop” from the main dash harness under the dash. This wire should be located under the left dash area and be part of the wiring harness marked "Trailer”. You may need to contact chassis or coach maker for the exact location and wire color if you cannot locate it. Often the individual wires have the application (Brake) printed on the insulation.
The SMI warning light is too bright to be used up in the coach, it will blind you if looked directly at while driving, especially at night. We recommend going to "Radio Shack” or its successor, "Best Buy”, "The Source” or similar electronics store and obtaining from their small parts cabinets, a small LED indicator light that you will mount on one of the switch blanks on the coach dash. If there is no blank then choose a suitable location and drill a small hole to mount it on the dash. The wire from the trailer harness is connected to the positive side of the LED and the remaining side is connected to a good ground. The final step is to use a label maker and print a "Towed Brake On” label which can be applied to the dash in the area of the light. If you want to use the SMI supplied light then mount it so that the light emitted from it is visible but not aimed at your eyes while driving. All you want to confirm is that the brake is on in the towed vehicle.
As of this writing (April 2015) SMI is phasing into production a revised brake cylinder that contains a magnetic switch that will replace the wiring connection to the towed vehicle’s brake switch. Use that connection if your unit comes with this updated activating air cylinder.