The trailer lights work when the cars park are on but wiring in is ok and uses a 7 pin plug wired to left tail indicator light?

There are a few reasons why your trailer lights may work when your car's park lights are on, but not when you connect them to the 7-pin plug. Here are a few things to check:

1. Check the 7-pin plug. Make sure that the 7-pin plug you have connected to the left tail indicator light is wired correctly for a trailer light application. There are seven pins on a standard 7-pin plug: two for ground, two for running lights, two for brake lights, and one for turn signals. Check that the wiring is connected properly to each of these pins.

2. Check the wiring in your car. Once you have verified that the 7-pin plug is wired correctly, check the wiring in your car that connects to the 7-pin plug. Make sure that the wires are securely connected to the proper terminals and that there are no breaks in the wires.

3. Check the fuse. There may be a fuse that protects the trailer light circuit in your car. If this fuse is blown, it will prevent the trailer lights from working. Check the fuse box in your car and replace the fuse if it is blown.

If you have checked all of these things and the trailer lights still don't work, you may need to take your car to a qualified mechanic or auto electrician for further diagnosis.