Trailer lights always seem to be out, or intermittent, or making the turn signals blinks too fast. It's a non-stop case of whack-a-mole to keep your trailer lit up properly, here's some advice on how to reduce your maintenance investment.
There are as many as seven electrical circuits in the standard automotive trailer plug, but don't assume that just because your trailer has a plug and your truck has a socket that all is good. Just for openers there are 4-, 5-, 6-, and 7-pin plugs.
Trailer lights are required by law: you'll need three kinds of lights.
The most basic is running lights, which are required to be on anytime it's dark. The location, color, and brightness of these lights are fixed by regulation: you'll need three different colors. The main running lamps on the rear, as well as the side marker lamps, must be red. Front marker lamps and side marker lamps must be yellow. This is so other drivers who see your trailer from the side can see how long it is when it's still too far away for their headlights to illuminate it. And you'll need a white license plate light, of course
At the rear, you'll need brake lights, which must be red. Turn signals can be red or yellow. Trailers don't often have yellow turn signals like cars, because they use a single two-filament lamp to keep thing simple. One filament is for the running lights, while a brighter filament is for the brake and turn signals. This bulb lights up for the brakes, and blinks for the turn signals.
Any trailer you buy will have lights installed already that meet the letter of the law. Consider upgrading to a more weather-resistant type of fixture if you have corrosion problems inside the socket. Because trailers often spend long periods without being used, moisture can creep in and corrode the connections. This goes double for boat trailers.
Also, consider upgrading to LED trailer lighting. Because LEDS are very long-lived, they are often sealed into the lighting fixture, without a socket to corrode; they should last as long as your trailer.
Another advantage to LEDS is reduced electrical consumption. They draw a small fraction of the amperage needed to illuminate a conventional incandescent bulb. If you have a large trailer with a lot of lights, this can reduce the strain on your tow vehicle's charging system.
There's another reason to change to LED lights. If you add a set of extra incandescent bulbs to the turn signal flasher of many cars, it will blink far too fast because of the extra current draw. Sometimes you can just change the flasher relay for a heavy-duty one, but many newer cars use and integrated lighting module that isn't compatible. Using LED lights, with their low current drain, skirts this issue.