Tail lights just up and died...

There is a tail lights fuse for the truck, but not for the trailer.
First, check that your left tail light on the truck is working, if not fix the problem (new bulb, bad contact, fuse).
Then the obvious suggestions regarding the trailer are: Check the tail light portion of the bulb to be sure it is not blown. If good (best to put in new one to be sure), then check the wiring from the front of the trailer all of the way to the non working bulb, including every connection and at any place it is secured to the trailer frame. If all is good with the trailer circuit and bulb, then check the truck's trailer connector, including any adapters. If by chance the truck is wired with a flat 4 wire harness the problem may be solved (but usually the blinkers are the problem). Modern trucks require a 5 wire harness which self adapts to the standard 4 wire trailer circuit. Check for dc power (12 volts) from the truck's female connector. One wire will be ground, others will be taillights (turn on headlights), brake lights (have someone press brake pedal), and blinkers (turn on switch and blinkers).

There has to be a break in the wiring, assuming you also replaced the bulbs when the taillight was replaced.

I guess this means your tail lamps for just running are out. Start with fuses, then bulbs, then light switch.

Relay switches are sometimes located in the fuse panel box or under the hood of the vehicle, look for a 12 to 14 inch long black box, you will have to take the top off and the relays and fuses should be there. Make sure the wiring connections to the lights and other components having problems with have good tight connections. Hope this Helps.

You will need to check wiring at the sockets, turn lights on and remove the tail lights from tail lens and wiggle bulb to see if they will light up . this is to see if its a connection in the socket.

Another area to check is to open the hood and check the wiring going to rear lights, look toward the driver side trunk hinge and check for broken wire or wires. this is a common area for wire damage. look closely.

If you put the tester on the outlets and it lights up properly but the bulbs dont, you have already concluded that the wiring isn't the problem. unless of course the wires are corroded and simply putting the bulb in shorts it out. Have you tested the bulbs?

If the 3rd break light on the rear of cab works then check the panels where the bulbs snap into make sure there is no corrotion if so replace the panels you can get them at any auto parts for $20. to $40 each.

hope this will help.

most probably a ground issue, common

It is a dual element bulb. Sounds like one of the elements is blown. Try replacing the bulb.

you have a bad headlight switch or a bad conection at the headlight switch