2004 f250, backup lights not working, fuse ok, bulbs ok I can't locate the Backup light switch to test it. Is it under the dash or on the transmission?
Also no power to the 7 pin trailer connector? I can find the switches for sale, but can't find where it's mounted at.

 

Welcome to the site
Guessing it’s an auto, If so, the reverse lamps activate through the MLPS switch/sensor on the left side of the trans selector lever.

 

It is Auto, There is nothing that looks like an MLPS. It's a 6.0 Diesel and the only thing on the shift lever on the transmission is the gear select cable from the shift lever under the dash!

 

The sensor is on the inside of trans for the 5R110.
I can help you with some pin out testing to trouble shoot the non op reverse lamps if you need.

 

there is an electrical plug near the top of the trans close to the engine,3 wires I don't see any other electrical connectors. Is it behind a cover?

 

Sounds like your looking at one of the speed sensors. The main harness connector is on the right side of the trans just above the pan level. The internal harness feeds the sensor.

But, the way the TR Sensor works is the PCM receives a freq/duty cycle or signal per each given range selected. The PCM interprets the ‘which’ range it is like in reverse, and then commands the reverse lamp relay in the Central Junction Box to close (PCM grounds the relay for control) flowing current out to the lamps. Not quite like the NS/reverse light switch of yesterday.
So,, before condemning the range sensor, make sure the #8 15amp fuse has proven voltage there, and if so swap the reverse relay with another and see if it then works. If still no, removing the relay and check for ‘switched power’, ‘Hot at all times’ would be the first step. If present, then check for an ‘applied ground’ from the PCM while reverse is selected. If the ground is missing, possibly the Range Sensor..
You could also test for any harness connector issues aft by using a test lead (fuse protected to 15 amps) and insert it into the relay connector (leg #5) and with the key cycled ON the reverse lamps would illuminate without the relay, gear selection, and PCMs control.. Making absolute sure your testing on the correct leg.,

 

If the fuse proves hot, and if swapping the relay doesn’t work, I can send you the connector view for a few simple tests, and with 12v test light and a DVOM meter, diagnosing or proving out the Range Sensor before swapping it out is a better bet..

 

I think there is only one relay like the BUL relay. Sounds like the FEDERAL GOVERNMENT DESIGNED THE BACKUP LIGHT SYSTEM !!!