2004 Horn Wire

Getting into that fuse box is quite a chore, and there might be some diagnostic opportunities you haven't exhausted yet to locate just where the fault might be. Do you have access to a multimeter?

 

Are your horn wires showing voltage when the horn button is depressed

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If you find the fault anywhere outside of that fuse box itself, you've got much less of a chore on your hands.

 

Another possibilty is that the contacts of the relay itself are bad. This is assuming that the horns actually work- been bitten by that. Maybe try swapping a relay for another one (that has the same part number). My next step would be getting a wiring diagram and testing at the relay socket. I would check to see if the terminal and wire that goes to the horns is open or intact.

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Yeah I'd be looking right on the other side of that relay before it gets out of the box too far to be deep in the wiring in the front end of the car I bet it's right there at that relay by the looks of things in these Priuses they take on moisture and water like Mom watering a plant it's unbelievable

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Me too. Definitely test that the horns actually work before doing anything else. Unplug and put 12 volts across each. If they don't work, give them a hard tap a couple of times. I had a horn which "stuck" but worked fine again (for many years) after that treatment.

Then look carefully at the connectors on the horns and whatever plugs into them. If anything looks at all dirty clean it up with some contact cleaner. If there is corrosion which won't come off on either side replace either the horn or its plug.

The instructions here may be helpful:

Replacing Horns on 06 Prius | PriusChat

If after all of that, and after you replace the relay with a known good one, and it still doesn't work, then you may want to borrow or buy a tool like this:

https://www.testequipmentdepot.com/klein-tools/datacomm-testers/tone-probe-testers/tone-probe-test-trace-kit-vdv500-705.htm

to trace the path from one or both ends: relay output (with relay not present) and horn input. If you are lucky the break might be some place that you could reach easily. However, it could also be buried deep in a cable bundle which is only accessible after much disassembly in the engine compartment. Still worth looking, though, as the failed horn might indicate rodent damage, and you could have other wires gnawed through at the same spot that you just don't know about yet.

If there is a break and it isn't any place convenient, just run new wires directly from the relay output to the horns over any convenient path. I would be tempted to get a couple of horn connectors cut off a harness in a junkyard and splice onto those, leaving the originals hanging. Then if it is ever convenient to fix the broken wire(s) in their currently inaccessible location while some other work is being done, you can remove the new wires, and put it all back the way it was supposed to be.

 

Some plate type horns have a set screw you loosen nut after smacking on metal table to loosen vibe plates . Apply voltage and adjust that but to proper amp pull or make it sound correct tighten lock nut. Or replace Fiamm work well

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