Need help from the brain trust again
 

I'm trying to fix my trailer lights and can't get the taillights to work.
Flashers work OK on both sides.
Voltage at the taillight connector on the flat 4 at the back of the truck fluctuates all over the place. I plug the connector in and get the same weird voltage fluctuation at the trailer taillight unit. Truck taillights working fine. It's a 2017 Tundra with factory trailer wiring.

On another note, I made the mistake of putting new, cheapie light units on the trailer years ago. They are still bright and shiny inside, but they grab the bulb with such a death grip that getting the bulb out destroys it. Arghhhhh!!

Check the grounds. And buy a plug in tester.

This. The majority of electrical issues on cars and other items is a poor or just bad ground.

Does the voltage fluctuate with or without the trailer connected?

As above, I keep a plug-in tester in my tow vehicle. They are cheap. Also, IMHO, it's better to test with a test light than a voltmeter. It puts a small load on it, and you only really need to know on/off.

I'd also swap those 'cheapy lights' for leds.

Yup grounds.
Go to the plug at tow vehicle and see if you have power there . Check brake lights etc.
If you do, it is the ground.

Just went through this on my 2017 Titan.
Turned out to be a couple of fuses in the truck.

Both. I get the same behavior at the trailer light unit as at the flat 4.

This. I’m suspecting it’s something in the truck. I tested the trailer with a 12 volt mower battery and roach clip leads and the trailer lights work fine.