You can"t always use a test lite for checking on newer vehicles, but you can always use one to check fuses and for power to certain areas.
If you have one great, if not I would suggest investing in one because right now the first thing you need to check with the lighting problem is the fuses.
You just connect the ground clip of the test lite to a good ground and proceed to place the test lite probe tip on each side of the top of a fuse. If it lights on one end , it should lite on the other side also. If not, simple, the fuse is open and needs to be replaced. This takes about 1 minute to do maybe less depending on where the fuses are located.
If the fuses are all confirmed good, now you need to start tracing wires from the light switch to the tail lights You can also use the test lite for this.

 

 

I believe now you need to confirm power is coming out of the headlight sw . You can remove the switch by pulling out the switch stalk and depressing the little button on the bottom of the switch and removing the **** , then the switch retaining nut, (bezel) . Now the switch is accessible to do your testing. Just remember the headlights will be on while you are doing this and if you need to do something else you can always push the switch stalk (****) back in to shut the lights off until you are ready to continue.
The color of the wire that is supposed to have power at the rear tail lights should also be that color at the switch .Confirm power is coming out of that wire, which I believe comes thru the door pillar down the back of the cab and then thru a connecting plug then on back to the rear of the vehicle.. If that color wire is there confirm it still has power going out of the vehicle and work your way back confirming power is available until you get to the point where you lose it It could be at the rearmost area, but you need to confirm every section and you will eventually find the break.
When you probe the wire you should apply liquid tape over the opening to prevent corrosion from having a place to start.


I would never enter any off color or crude language and can only hope the asterisks are the procuct of a computerized program.

 

 

It would probably be a good idea to know what you did to get the tail lights "working".
Not knowing that just throws more mystery into the problem.
Did you do the checks suggested on my earlier post?

 

 

I don't remember if the parking lights (front) always stayed on with the headlights on , but if that is what you want just hook up the parking lights with the tail lights which go on any time parking or headlights are on. There was a plug that went on to the headlight switch, was that plug removed and wires attached individually? I should have a diagram for that system , will have to check.