There's got to be some smarties around here than can help. I have to pull a quad on a trailer tomorrow, and I borrowed one and need to get this sorted out kinda fast.

My car has had a hitch but never a trailer light converter (it has separate amber turn signals). So, I bought one and installed per directions. It's a 4pole flat connector, same as the trailer. Function tested it with multi-meter and it works exactly like it says it should. Pin 1=ground. Pin 2=constant (park). Pin 3=left signal. Pin 4=right signal. Pins 3 and 4 together, brake. Good to go.

I pick up the trailer and plug it in and whala, I have lights. But, one small problem. With headlights on, one side of the trailer's lights are on bright, constantly...as if the brake light is always on but just on one side. Other side works right. Upon closer inspection, I see that a turn signal or brake light does light up the little secondary bulb for turn signal, and does burn the second filament - it's just not noticeable. It's like the light's primary filament is just too bright.

I discussed this with my neighbor, he seemed to think I had bad wires, but I'm thinking the bulb is goofed up. I haven't switched bulbs to see if that's the problem, it got dark on me, but does this sound rational or am I overlooking something obvious?

Does the way the bulbs work is when you apply brakes or signal, the juice is effectively doubled by the 3rd or 4th pin hitting that bulb and thus lighting up the secondary filament to brighten that bulb? If so, then it seems it would have to be a bulb problem because any other wiring errors I can see would create a non-working light, not a light working too much.

Any advice from the experts?

check the ground on the trailer maybe has some corrosion.

Since you only borrowed the trailer, presumably for a short time, I wouldn't worry much about it.  Unless the cops in your area don't have much to do, improper trailer lights aren't real high on the enforcement list.

Every time I've bought a trailer with lights that didn't work right, they always get rewired.  A 25 ft. kit from WalMart is around $25 for all the wiring, flat 4 connector, 2 tail lights and 2 marker lights.  It only takes 30 minutes to an hour to install it, and it saves you a lot of headaches in the long run.

Ok, let's belabor the obvious for a minute. You hooked the trailer hitch up to the towing vehicle before plugging the connector in? Sounds like a ground deal to me, too.

NMSight

Most trailer light problems are due to POOR GROUND. Check the White wire on the trailer (should be secured with a screw near the coupler). If the screw looks rusty, replace it and the connector too.  Clean the plug on the trailer with sand paper .

Yep, check for bad ground. Bad grounds have strange symptoms. A flat four plug will ground through the ball hitch, make sure there's a good connection through there. As a test, try a jumper wire from the trailer to the tow vehicle. The lights might even out.

Yep, almost certain that your ground connection (WHITE wire) on either the trailer or tow vehicle side is not a solid one - and most likely it's on the trailer side. Without a good ground, the current has to travel back to the tow vehicle through the other bulb filaments. If it's a small filament, it lights pretty brightly; if it's a large filament, it just barely glows.

BTW, here's a handy little trailer wiring guide.

on the trailer side; the most comon for a 5 pin trailer plug with no electric brakes

white-ground
left-yellow  brown park- lamp
right-green brown-park lamp

Or you can figure the owner of the trailer didn't wire it to specs.