So, ive spent a few hours on my back, yesterday, and today. Wired in a junction box, a new 7 blade with backup alarm, replaced the right blinker fuse, ran a bale spike remote wire into the cab. Mounted some backup lamps, and work floods. All in all, i feel like i got a decent amount done. Couple things arent finished, pending random parts, but my backup lamps didnt come on (truck backups did light up) and my new backup alarm didnt beep, so i put the multi meter on the plug, and get a reading of 0.12 or less, on the 20v setting. Doublechecked the fuse, it has continuity, and appears normal. Blades are discolored, but the fuse loop isnt. Doing this myself, this was all key on, engine off,so i could put it in reverse and work. I did start it, to see if the alarm would come on, but it did not, and i couldnt meter it, for obvious reasons. My first thought was maybe that wire is about to break somewhere, and its barely connecting, but i figured i should ask before o tear something up, needlessly.

 

This is from the shop manual:

1. You already verified that the truck's backup lights work, so the issue is isolated to the trailer lighting system.
2. Next step is to make sure you have the five-pin trailer backup light relay installed in the little box under the hood near the underhood fuse box. My truck had never had a trailer plug connected before I got it and did not have either of the two relays that go in this box or the trailer light fuse in the fusebox.
3. Next, check the continuity between pin 87 on the backup lamp relay socket in the engine compartment (the pin numbers are marked on the relay) and the center pin of your 7 pin harness and between that pin and ground. Relay socket pin to center pin of the trailer socket should be <5 ohms and relay socket pin to ground should be open (>10k ohms.) If this is not true, you have a wiring fault between the relay box and the trailer connector that needs fixed. You will likely need to rig up some extension leads for your multimeter as the distance between the relay socket under the hood and the trailer plug socket on the back of the truck is about 20 feet on my truck.
4. If the values are as expected above, the next step is to make sure you have voltage (>10 volts) between pins 30 and 86 on the relay socket. If no, you have an open in the wiring supplying the relay box under the hood.
5. Last step is to check the resistance between pin 85 of the relay socket and ground. This should be <5 ohms, if not, the wiring to the relay box needs fixed. If it is <5 ohms, replace the relay.

 

I have found that over a certain amount of current draw, the trailer reverse circuit just can't handle the load. That is why I wired a dedicated reverse supply straight from the battery, fused of course, that is connected by a relay that the trailer reverse circuit controls.

As for the reverse alarm, always go with the Japanese talking version.

 

Ok, yay, but no.... so i got under the hood as instructed. Found the little box, check. 2 relays inside it, check.
Check under to see where the wires came into little box, found an empty socket, oh, make that 2 empty sockets... after scratching my head for a bit, i figured out that the little box had been bypassed. Ok, cool. Un plug the direct, add little box into the circuit. Alarm goes off. Sweet. Plug in bumper reverse lamps. They light up. Double sweet. Take the truck out of reverse... no change, wtf?!? Key on, so is the alarm, and lights. Swap relays, same result. Start truck, put in a forward gear. Still on. Is the transmission reverse switch stuck? Both relays faulted? Other ideas?

 

The reverse switch on the transmission isn't the issue if your truck's reverse lights go on and off when you shift into and out of reverse. You mentioned before that your truck's reverse lights worked properly, so the problem would be in the trailer backup light system. You then noted that the wiring to the relay box had been "bypassed" by the previous owner, so what I would do is check the continuity of the wires from the relay socket pins in the box as mentioned above. It sounds like the backup light wire is getting voltage all of the time when you un-did the previous owner's hack job. This may be why the previous owner "bypassed" it. I have found that if wiring is modified, it is usually because there was some fault somewhere that somebody didn't want to go through the hassle of diagnosing and fixing properly, so they cut or splice wires to eliminate the worst of the annoyance.

If that all checks out, then replace the relay. You would only need to change the one relay (#38), the other relay in the box is for the trailer battery charge.

 

This isnt resolved, its gotten more convoluted, actually.
towed last week, and had issues. Had dicovered a flipped wire that i had done. Fixed that, and figured id run without trailer reverse lights... halfway home, the whole trailer light system "failed" the trailer came unplugged, and when it was plugged back in, the brake box lit up, but no lights. At all. Freaky, and frustrating. I removed the relays that i had put back in, and the lights (minus the reverse) all started working again. Im not sure why work/no work/work... considering that the pigtail only has 2 relays, and shouldnt be effecting the lights themselves. Im going to pull the pigtail and check for shorts on it, while removed, since its been the only variable in this mess. Very much inclined to do what noob suggested and put a relay in on the truck lamps to feed the trailer.