So I've made a workaround, but it's not permanent. I've run a patch between the red/black wire just after the clock spring directly to pin 86 of the horn relay in the engine bay junction block. Horn works via the wheel just no door lock horn.
I've tested the continuity between the same red/black wire from after the clock spring to the ctm connector wire. I don't have any. Running a patch there did nothing, but as I read the schematic, it should be the same wire? Thats where my patch should work, but no dice.

 

86 or 85? 86 should be connected to the battery. Since the horn works when you press the button, I'm assuming that you typoed and actually have it on 85.

My manual shows this as a pretty simple schematic. Both the horn switch and the CTM apply ground to the red/black wire to sound the horn. On the relay, both 30 and 86 should be going to 12V, and should be hot at all times. 87A should be open, and if you check it you should see 12V on it when the horn is not sounding. Can you explain better how you patched between the CTM and the clockspring wire? Were I doing it for testing purposes only, I would slide a paperclip into the back of the CTM plug where the red/black wire enters the plug, making sure it contacts metal inside. I would then plug the plug back into the CTM, and attach a wire to the paperclip making sure that it is taped off so it can't contact anything. The other side should be joined in a similar matter to the red/black wire on the clockspring, or to 86 on the relay. The only way to test this would be with your alarm, as the CTM would only ground the horn relay when you lock/unlock the truck. Alternatively, you can unplug the CTM and just run a wire from the red/black wire on the plug to ground, and the horn should sound. If it doesn't, there is an open in the wire. It sounds like this is a problem anyway, since you say you don't have continuity from the CTM plug to the clockspring plug.

This is where it gets messy - physically tracing the wire. You would need to pull the dash and trace the wire from the CTM to the junction, and from the clockspring to the junction. From the junction, you would need to trace the wire to where it connects to the relay on 86.

Hope some of this helps.

 

I said the numbers wrong, OP is likely just rolling with it.

personally, I think that 3-way splice in the junction block has come apart. That's the only thing that would make sense for there to be no continuity among the three legs.

 

A very late update. I have been switched to night shift this week suddenly so this has sat on the back burner. sorry about that.
I made a semi permanent workaround for this by running a patch wire between the red/black wire coming off of the clockspring right to the same wire just before the large bolted connector in the firewall. This works well and gives me my horn back. However I discovered that this was not a great fix as I now have an ebrake and abs light on the dash and the Cruise is now disabled.
So I guess there's still work to do.
I'm inclined to agree with Magnethead. its the only spot that makes sense why there's no continuity on either branch of the red/black wire out of the clockspring.
What does that 3-way splice in the junction block look like, or whats the spot to look in? I think I have to do a proper repair or nothing on this one.
Unless there a way to avoid these new faults, but that's fighting a problem I made, I'd like to fix the original problem if we can.

Thanks again for everyone's help with this, It's been a massive help and got me 90% of the way there!

 

If you remove the jumper, do the codes go away?

I wish the cab connector pin location was listed in the FSM on those diagrams.

I just realized. The junction block in the FSM is the dash-side unit, NOT the underhood (PDC). Not sure how I missed that earlier.

So the horn in my FSM is top center-left relay. The diagrams show connector 8 pin 7 is clockspring and connector 8 pin 8 is CTM.

Connector 8 is the brown middle connector of the left column.

Again, remember my FSM is for 2001.

https://photos.app.goo.gl/G98rD3Hsw4E7cQBY6



https://photos.app.goo.gl/CQruvPaZxJAVAs3p9



https://photos.app.goo.gl/q4qo7QsNwnpD95GV8

 

Unfortunately with the jumper disconnected the codes and lack of Cruise do not disappear. What threw me about the original fix, when started, I've always initially gotten the abs and ebrake light on, only to turn off after a few seconds. So the same happened when I did the patch repair. When testing today, the ebrake and abs light do not come on until i hit a certain speed, likely when the cruise can be set. I think its between 37 and 50 mph?
I checked all fuses, all are good. What have I blown?
I'm unsure how to proceed from here. I'd love to fix the new issues and start fresh with the old, but I'll take any advice you can give.


So I've doubled my issues. Time to hit the casino and double my money.

 

It's possible that when you were into the wiring to add the jumper that you left a plug off somewhere under there. When you're concentrating on fixing a problem, your mind doesn't always note what you had to remove to get to the problem. I fixed a clockspring on my old Corolla, and forgot to reconnect the cruise plug under the steering column. Had to pull it all back apart to reconnect, cruise worked fine after that. Hopefully your new problem is just an unconnected plug.

The ebrake and ABS lights coming on for a few seconds at startup is normal, it's a light bulb test.

Since we know that the black/red wire only goes to three items, the clockspring, CTM and horn, the easiest fix would be to just run a new wire to connect the three and cut off the existing wires. I like soldering then covering them with heat shrink. This would let you splice into the existing plugs and just replace the actual wire, and you can put the splice somewhere you can find it again in the future like at the horn.