Kenworth T680 taillights not working

I have a 2019 Kenworth T680 where the taillights on the rear of the truck will not work as a running light - the brake lights both work - the turn signals both work - the 4 way flashers both work - they even flash in test mode - however they do not work with the headlights on or the marker lights on

any ideas?

 

i pulled the cover off the fuse box and read the labels but not finding one that read just taillights

 

I found the one that said park lamp - it was a 10 amp fuse ... all I had was a 30 - I switched it out but still nothing

 

so when I checked it with a volt meter I did not get 12 volts on the running light ... I only got like 9.4 - I was thinking it was because its a dimmer light than a brake light so that might be right - I put the new lights in and just by looking at it they still do not look like they are working ... but they are ... just very very dim - to the point you have to put your hands around the light to see them working

any ideas

 

I fixed mine ... unconventionally but it works ... I ran a wire from one of the marker lights on the front along the frame all the way to the rear of the truck ... figured out which was the running light and cut the wires and tied them to the line I ran from the front