Question: Is it difficult to adapt a motorcycle trailer to work on a car as far as lighting goes? I know the trailers are usually five-wire and my car uses four. How does that work?
Answer: It depends. Often it’s not too tricky, but sometimes the setup throws you a curve. The reason a conversion is necessary is because, as you say, a motorcycle trailer usually has five wires and four-wheelers use a four-wire setup. Motorcycles, unlike cars, has a brake circuit separate from the bike’s tail lights. Most cars/trucks have brakes and tail lights that share the same bulbs.
Easiest: If you haven’t yet ordered your trailer, ask your manufacturer if the trailer can be wired for a four-wire system. Some accommodating folks will do this for you just for asking. Put a four-wire harness on your bike, and you can swap the trailer between your bike and car with no conversion.
Simple: If your motorcycle trailer has turn signals with red lenses, you’re in luck. All you need to do is hook up the ground, running light circuit, and two turn signal circuits on your trailer to a four-pin plug and ignore the wire for the brake circuit. This will plug up and work with a car/truck with no problem. You can actually rig up a converter to go between the existing plug on your trailer and a four-wire plug to match the car side without doing any re-wiring on the trailer or your bike. When you want to tow with your bike, just remove the converter.
Less Simple: If your turn signals are amber, you can’t use them as turn and brake. Brake lights must be red. The next simplest thing is to source red lenses for your lights. Many trailer makers use off-the-shelf lights which are available in either color.
Harder, but doable: If you have a trailer that uses proprietary turn signals, like those that match the Honda Gold Wing, you have more engineering to do. That’s because neither the old style (01-05) nor new style (06+) offer tail lights with red turn signals. In this case, you need to do some re-wiring. The easiest way to accomplish this is to move the turn signal wire down, replacing the brake wire on both sides. This is a permanent change, so you’ll want to confirm that by doing this you will still have running lights. You will also need to put a four-wire harness on your bike to run the new system. You are not building an adapter like the “simple” process above, you are making a permanent change to your trailer.
As always, with any wiring project, test your work before you make it permanent, and always do a shakedown run. Don’t do this the night before you take a long trip! If you aren’t comfortable making wiring changes, enlist the help of a buddy from your club or if someone at your dealership is knowledgeable about trailers, ask them for help.
Got a question about trailering? Feel free to send it to firstname.lastname@example.org and I’ll try to answer as best I can. Thanks!