Trailer Marker Lights

Sometimes you need to take a small standard head screwdriver and open up the pins a little bit.
Dont push to hard or deep , you can break one side off. Since you've cleaned them it won't be hard to see the slit in the pin.
Just had this happen to a trlr I picked up two days ago. I hooked up and only had right side blinkers , took my little screwdriver and spread the pins a little and bingo bango problem solved.
Just the tip of the driver , your only going to open them about a 16th of an inch.
If you don't have a small one use a knife blade.

 

Checked the earth going to the plug yet?

 

Does the tail lights at the tractor work? Marker lights on top of the cab? Do you have lights power at the back of the cab where the pig tail cord originates? (using a tester) I forget which pins you should look at, but there should likely be at least 2 hot pins there with lights on, then turn lights off and see if one of the hot pins is no longer hot. Then trouble-shoot further to verify the pigtail cord is good.

If you've changed the cord with same negative results, could possibly be a bad "marker lights interrupter" switch in the steering wheel or a bad relay in the fuse panel.

 

First step in that sort of thing is always to put a test light on the pins to find out if they are hot or not. Rather than wasting time trying to clean connections, spread pins, putting wd40, etc, find out if there is power to the pin. Obviously you aren't getting power to the trailer, so put the test light on the pin and on ground and start working your way forward from the pigtail and see if you have power somewhere.

 

My money would be on a bad relay [from the interrupter switch] ... probably in amongst all the other fuses at the main fuse panel