Sigh.... If its not one thing its another. I am placing  this here because boats have trailers and trailer owners who use them.

My dilemma. No lights in back. Took multi-meter checked the connection  while connected  to vehicle It has power and blinkers as well, bulb are burned out.

side lights  work as well.too.

There is one light a white one up by the  pulley that is for the bow eye strap. I've no idea why this light is there. It used to work now it doesn't.  So I look at the bulb....its ok. Check if the fixture has power ....nope. I start unraveling electrical tape and find some weird connector I've never seen before. basically. Also the black (which is the power comes from the light and hooks into a red wire from the trailer. The black goes from the to the scoket (poer) and the splits into a little box that is connected to the plastic light cover (ground of some sort) So I basically  I cut it out the weird connector and the weird possible ground and splice the  black light directly to the red lead that powers it fron  ( I checked it to see if the red had power and it does.  In the light I found a white wire  inside the light that was not doing anything it just stopped and had no bare wire at all. I used this as  the ground since it was connected to the fixture but not the socket.  

So I hooked it up checked that the fixture itself in not hoy=t incase I messed something up It wasn't.

I connect everything plug the trailer to the car the light works. Yeah!   Tape everything  up, put the cover on the light and once again plug the trailer in and.........No lights at all WTF! none not even the ones that were working!

I hate this crap!

Thoughts?

did you blow a fuse on the vehicle? check and see if you have power at the trailer plug other than that check the ground on the trailer something had to happen to where all the lights Quit working.

I'll have to check but How could that blow fuse? It one tiny little light. UG!

I heard a rumor some years ago about a guy up in Michigan who had a boat trailer and ALL the lights worked as intended.This has never been confirmed though.

from Outdoor Life a long time ago..........I don't recall the author's name .....McManus??

so the little connector (they are blue) are junction boxes. and the other weird thing that the black line split onto near the light was a switch...DUH. So I did wire id correction Black (power to red (which was split off of one of the brown running lights at a junction bos thing) and whit to ground (although the light was ungrounded. I suppose it uses the trailers ground. Hmm. Can to have more than one ground?

ETA. Got a thing to test the wire harness. No lights from it so there is a blown fuse some where. This is pretty annoying since I had it all working with no problems and then nothing.  Now I have to find that fuse.

We gave up on the boat trailer lights, because they really aren't waterproof, aren't reliable, and started over on our small trailer.
I don't know if the new LED taillights are any better after they are submerged, but they are more expensive, than the old bulb lights.
Our boat is an old aluminum 14 footer.
My Dad finally built an L shaped wooden bracket and drilled a holed in the gunwale (top of the side rails) on each side, and we mounted the lights on this bracket across the width of the small boat.
It's secured by a bolt from underneath, and a wingnut on top on each side for quick removal.  Some plastic clips are double faced taped inside the boat, to hold the wire while traveling.
The lights never get submerged, and they are up on top of the boat, to be better seen from people following at night.
It takes but a couple of minutes at the landing, to take the bracket with the lights off.

The blue "junctions" are called Scotch Locks. I hate them with a passion and eliminate every one I see. While they are fine for a speedy trailside repair, they have no business in a permanent installation.
The easiest thing to do is buy a complete light kit including the wiring harness. Strip out all the old wiring (or leave it there) and install the new kit. You probably have a short to ground somewhere in the harness.
Accessory lights, like the one you describe, sometimes have two grounding options. One ground is usually direct to the socket and uses one of the attaching screws to make the connection. The other ground is a wire to facilitate mounting it to a plastic or fiberglass fender. What you may have done is attach the hot wire to the "other" ground.
The blown fuse will be in the vehicle. Look at the tail lamp/running lamp fuses.
As far a blowing the bulbs when submerging them, just unplug the trailer harness before you prep the boat for launch. This gives the bulbs time to cool off before going into the cold water.
Coat the light sockets with grease to help prevent corrosion and drill a few small holes in the bottom of the light for drainage.

ok. All the lights are working! So right now I am that mythical guy in Michigan!

Thanks for all your help guys.

Something that caught me a couple of times is many small trailers use the hitch ball itself as the ground between the trailer and the tow vehicle.

If the trailer isn't used often or at the begining of the season a bit of rust/corrosion will make this a iffy thing.

I found that sealed lights just hold in the water . The type that are like a inverted box with the bottom open will allow air in to dry things out and seem to work fairly well.

Most all trailer lights I have seee are cheasy as all get out and are pretty much toast after 5 or so years

The absolute best way to do it is put the lights up out of the water up on PVC boat guides. Get an inexpensive 50' orange 3/14guage extension cord and run a length down each frame rail from the connector uninterrupted up to each light. Solder, silicone and heat shrink tube so you get a blob of silicone out of both ends of the shrink tube on every splice at the lights and connector. Do it the right way once and you will never have to screw with it again. Ever.

thanks I will do this. whe it is time to replace these lights which will be this year.

Many boat trailers actually don't have a ground connected through the jack and socket where you connect the trailer wiring to the vehicle , they depend on the trailer grounding through the ball hitch . Rusty ball or hitch and you don't get a good ground . The vehicle socket has a ground wire but it is many times not actually bolted solid to the trailer frame or is rusted .