BTW -- Just a point of interest for those out there using older trailers with newer towing vehicles...
Most boat/utility trailers use four wires:
- right turn;
- left turn;
- tail; and
- ground.
The stop light tie-in occurs in the towing vehicle's turn signal/hazard switch. One of the problems that occurs with many newer (towing) vehicles is that there are stop lights that are separate from the turn signal lights.
Most older vehicles used a single dual-filament lamp for the functions of tail light, stop light, and turn signals. The lower candlepower filament was used for the tail light function, while the brighter filament was used for both turn signal and stop light/hazard functions. Thus, if the driver applied the brakes with the turn signals "off", the current from the stop light switch was routed through the turn signal/hazard switch to both rear lamps. If the brake was applied with the turn signal "on", the stop light current was routed via the turn signal/hazard switch to only the rear lamp on the side opposite the active turn signal.
Newer vehicles generally have the stop light and turn signal/hazard functions provided by separate lamps. Thus, the combine/separate function of the turn signal/hazard switch is no longer needed or present. As a result, you end up with five wires from the towing vehicle (r/s turn, l/s turn, tail, stop and ground) instead of four as described above.
This leaves you with two alternatives:
- add additional lamps to the trailer, or
- use a conversion device to selectively combine the stop and turn circuits.
Adding additional lamps is not a bad idea if space on the trailer allows. it makes the trailer more visible, and thus can improve safety. It also increases maintenance requirements. if doing this, you can simply add two more lamps -- one to each side. These lamps can be of either single or dual filament type, and either way should be mounted inboard of the existing lamps. The existing lamps should be used for the turn signal and tail lamp functions; the added (inboard) lamps should then be used for the stop lamp function (and tail lamp as well if dual filament lamps are used).
Using a converter eliminates the need for additional lamps on the trailer. Converters are commerically available at most trailer and/or boating supply houses. Or... you could build your own using a circuit
**broken link removed**. I don't recall the original source of this basic design, but I have updated and clarified it, and I have added the standard trailer wire color codes as well.