Dual Loud and Quiet Horns on a Toggle Switch

Maybe my miata’s best party trick is the dual horns. With the flip of a switch it can alternate between the cute OEM horn and the loudest horns put on a production car. All cars should have this; apparently Bentley included this feature in some models as the “Town & Country” horn. The video speaks for itself.

The first step of this project was to research which cars at the junkyard would have the loudest horns to salvage. I found a forum post that recommended finding the largest American land yacht possible, and that means 80’s Cadillac. For years Cadillac was putting four(!) horns in the grill of their full-size luxury sedans.

I fabricated some metal bars to mount these horns to existing holes in the cowl in front of the radiator.

The wiring I created for this project is a bit hairy. I wanted to created two horn modes with a toggle switch. The first setting is an even quieter than normal OEM Miata horn, and the second setting is all four Cadillac horns plus the Miata horn. In retrospect, the Cadillac horns completely drown out the Miata horn, so the extra wiring complexity to make that work wasn’t worth it.

The actual wiring work to make this circuit was roughly like this:
1. Add a new fused power source (in the fuse box if you’re feeling fancy) that sends power to all four Cadillac horns through a 12V automotive relay.
2. Find the original horn mounted behind the front bumper and disconnect the wire that powers it. Instead connect that horn wire to the triggering circuit of the automotive relay.
3. Run the negative side of the new horn triggering relay all the way back to a toggle switch in the cabin that connects/disconnects it by grounding to the chassis.
4. Add two possible current paths to the original horn – one that comes from the same original wire, but with a resistor to eat some voltage before the horn, and another that brings power from the Cadillac horn relay. Both of these paths need one-way diodes so the electricity doesn’t get confused and go the wrong way.

The end result is that pressing the horn on the steering wheel always triggers the OEM Mazda horn relay, but that horn now has an in-line resistor that makes it quieter. When the toggle switch is flipped, pressing the horn also triggers the new relay, which powers all four Cadillac horns and bypasses the resistor on the OEM Mazda horn.

I used https://www.falstad.com/circuit/ to figure out the circuit. That’s how I realized I would need the diodes, without them the loud horns could get stuck in the on position 🤯.

The body of the two-pole toggle switch in the above photo was too large to fit in a dash blanking plate, so I switched to a smaller three-pole toggle switch (but still only used it connect/disconnect ground to the horn relay). I used two 1 Ohm resistors in parallel to get the OEM Miata horn to the quietness I wanted.