Scott, You'll need to use the existing horn lead as the "actuator" and tie it to terminal 86 on the relay. Pin 30 of the relay goes to (+) terminal on the battery (fused of course!). The horns should be well grounded (to their mounting points) and the horn "hots" go to pin 87, and 85 goes to Ground as well.
lee and Lwin, the light gauge wire is the downfall of the OE wiring, so it's best to pull fresh battery source straight from the Battery. Or, maybe yours truly is just being anal about the whole thing!
Glenn
Edited for clarity, and safety! Thanks Lwin!