Very straightforward upgrade to replace the stock trunk mounted single tone horn with a dual tone setup in the engine bay. I am also looking into tapping the signal wire into the stock horn wiring to eliminate the need for a second set of horns.

Will work for any B5 or MK4 VW/Audi, or any vehicle that uses a simple 12v pulsed alarm.

Parts Needed:
Dual Tone Horns (can use any dual tone horns, I went with OEM Audi/VW to match the actual horns or you can use a 12v siren)
Relay
Female Blade Connectors x4
Male Blade Connectors x4
2x 16 feet lengths of 14 gauge wire
3x 1 foot lengths of 14 gauge wire

Install:
Remove the stock horn from its mount in the rear left of the trunk behind the carpeting/toolbox area.

Unplug the connector and plug in the male ends of the 16 foot wire

Feed the wire along the door jambs to the ECU Box and out the top

Splice the horn wiring together and crimp male blade connectors on

Add a 12v source to the battery, connect to the relay (Terminal 87a)

Add a ground wire and plug into the negative terminal on the horns

Plug in the horn source wires to the relay (Terminals 86/85)

Plug in the horn 12v to the relay (Terminal 87)