I just bought a used jet ski on a trailer that needs a bit of work. I installed new tail lights and ran new wiring but have one problem. The brake lights and turn signals work fine but for some reason the tail running lights do not. As far as I know the lights are not grounded right now. I don't know how you'd do this or if it would solve the problem. I'm sort of a rookie wiring guy. Can anyone think of anything I can do that would solve the problem? If is is a ground issue, I have 4 sets of wires coming back from the trailer connector that connects to my truck and 2 of those go to one trailer light and the other 2 go to the other trailer light. Hope all this helps. Thanks.

 

If your stop/turn lights work, you have a ground.

Check your truck fuses, fuses, fuses...there is a fuse specifically for trailer running/park lights.

 

check the bulb, they are double filament i think

 

You are correct they should be a double fillament bulb or multiple intensity LEDs.

If they were brand new lights and wiring as mentioned turn/brake lights are working and no running lights were working a fuse is a more likely fault....or poor/incorrect wiring.
I was giving the benifit of the doubt that all was installed correctly.