Horn relays are available through any auto parts store. Relays generally come with a wiring diagram.
Most relays have 4 terminals, numbered 85, 86, 87, and 30.
Terminal 85 goes to ground. The mounting bracket on some relays is terminal 85.
Terminal 86 goes to the switch that controls the relay.
Terminal 87 goes to the device the relay controls.
Terminal 30 is the fused wire from the battery that provides the power for the device the relay controls.
Normally.
Unfortunately, TWs are not normal. Actually, horn circuits in general are not normal. Horn buttons only have 1 wire? What? How does the electricity flow through a switch with only 1 wire? Simple. the other pole of the switch is grounded. Horn relay control circuits generally switch the ground, not the hot. Still, it's an easy fix.
Install the fused (15 amp), red 12-gauge wire from the battery to Terminal 30. When you attach the connector to hook up to Terminal 30, include the 12-gauge main wire and a 16-gauge wire about 3 inches long to which you affix a terminal on the other end to plug into Terminal 86. What you've done is jumped between Terminals 30 and 86, which saves having to run a separate lead to terminal 86. I recommend buying an inline fuse (10 amp) holder with pigtails and using it for the jumper, which will provide some measure of safety against a short in the switch and related wiring.
Unplug the stock pink wire from the horn and hook it to Terminal 85. This wire goes to the horn switch, and the other pole of the horn switch goes to ground, which, when pushed, will complete the control circuit of the relay and cause it to activate the controlled circuit.
Run a new 12-gauge red-with-white-stripe wire from the horn to Terminal 85.