Roadmaster Flat-Tow Wiring: 154, 155 & Smart Diode Parts Guide

When towing a car behind a motorhome, the car's rear lights need to show the motorhome's running, brake and turn signals. Roadmaster offers several different ways to do that. The right kit depends on how the vehicle's taillights are wired, whether they use LEDs or incandescent bulbs, and whether you want to use the vehicle's existing lamps or add separate towing lights.

Compare Roadmaster lighting kits

Standard diodes versus Smart Diodes

Standard Hy-Power diodes let motorhome lighting signals power the towed vehicle's original bulbs while preventing current from feeding backward into either vehicle. Roadmaster's 154 kit includes four diodes, a 27-foot four-wire harness and a 6-foot four-flat cord to connect the two vehicles. The wiring requires access to the vehicle's rear lighting circuits.

Smart Diodes are designed for approved vehicles whose taillights use multiplex or variable-voltage signals. In those vehicles, a conventional diode may cause the running lights to illuminate at full brake-light brightness. Roadmaster 153789 contains two Smart Diodes for eligible vehicles with LED bulbs. The 152789 kit uses the separate wiring arrangement specified for compatible Jeep LED systems. These two kits are not selected by appearance alone.

Both 153789 and 152789 include a 27-foot four-wire vehicle harness, but the motorhome-to-car electrical cord is sold separately. Check the connector fitted to the motorhome and the front of the car before selecting that cord.

Compare flat-tow wiring kits

154 — Universal diode wiring kit

Four standard Hy-Power diodes with the 27-ft wiring harness and a 6-ft 4-flat connecting cord.

View Roadmaster 154

155 — Bulb-and-socket lighting kit

Separate 1157 bulbs and sockets that isolate the towing lights from the car's original light wiring.

View Roadmaster 155

153789 — LED Smart Diode kit

Two Smart Diodes for approved vehicles with multiplex/variable-voltage LED taillights.

View Roadmaster 153789

152789 — Jeep LED Smart Diode kit

Jeep-specific Smart Diode wiring solution for compatible LED/multiplex applications, including certain ParkSense systems.

View Roadmaster 152789

157 — Auxiliary LED tow light kit

Separate external LED strips and a dedicated harness; avoids cutting into the factory lamp circuitry.

View Roadmaster 157

Why choose the 155 bulb-and-socket kit?

Unlike a diode kit, Roadmaster 155 adds a second bulb and socket within each applicable taillight housing. It keeps the towing light circuit separate from the car's original taillight wiring. The kit includes two 1157 bulbs and sockets, a 30-foot harness, a 6-foot connection cord and wiring supplies. Each socket requires a 1-inch mounting hole, and the bulb extends about 1⅞ inches past that hole. Measure available room inside the taillight before drilling; not every housing accepts an additional bulb.

Replacement diodes and converter parts

For conventional Hy-Power wiring, Roadmaster sells replacement diode quantities separately: 790 single diode, 792 two-pack and 794 four-pack. A separate 790-25 bulk pack is also listed. These are standard diodes; they do not substitute for Smart Diodes on a vehicle requiring variable-voltage control.

The Roadmaster 732 Brite-Lite converter changes a separate brake-and-turn source to a combined brake-and-turn output. It is directional. If the motorhome's trailer socket already produces a combined signal, a 732 may not be needed. Test the actual socket or use the motorhome manufacturer's wiring diagram before adding a converter.

Before installing or troubleshooting

1. Identify whether the motorhome provides combined or separate brake and turn signals at the towing connector; the appearance of its rear lamps alone is not enough.
2. Verify the car's lighting type and check Roadmaster's exact application. On a multiplex vehicle, make sure the Smart Diode kit and bulb type are correct.
3. If the vehicle has a supplemental braking system, review the wiring instructions for brake-light override or a required brake-light relay. That choice depends on the vehicle.
4. Follow the manufacturer's wire routing, grounding and circuit-test instructions. Keep harnesses clear of heat, sharp edges and moving parts, and test every lamp function before travel.

For a purely external lighting option, see the Roadmaster 157 auxiliary LED light kit. Its light strips use a separate harness and adhesive mounting rather than the car's original lamp connections.