trailer running lights out but flashers brake and clearance lights still work

Might be the pigtail connector on the tractor. They get worn down and don't make contact in the pigtail cable, but will still show power on a multimeter. Running lights are on a different pin than the lights that are still working on your trailer. Give the pigtail a good wiggle on the tractor side while it's connected and see if you can get those lights back. If it is the issue, you can limp it down the road with a bungee pulling the connector to the side, but it's a $15 part and takes at most half an hour to change out if it has the screw terminals and you have a lot of corrosion to remove in the process.

 

Have a 2016 Freightliner Cascades Trk keeps blowing fuse under the hood for tail lights and running lights on bottom still brake lights and blinker?

 

Changed pigtail but was not the problem

 

I can put fuse in and have running lights till I turn headlight's on then it blows fuse

 

Sometimes the pins on the trailer connector get nasty and don’t make a good connection. Try cleaning the pins with electrical cleaner or starting fluid then wipe out any fluid that didn’t evaporate.