Car Alarms: Horn wiring for alarm system, alarmtek, negative input

Question
I am in the process of installing a keyless entry/alarm system in my 1997 Ford F150 pickup. Everything's hooked up and working except the siren/horn. According to my wiring diagram and other sources, my horn wire is a dark blue wire located at the steering column and requires a negative input to activate.  However, the directions for my alarm system say that it provides a low current (+) output and should only be connected to the low current (+) output from the vehicles horn switch. I'm confused. Do I need to wire a relay to reverse polarity to the horn wire? Basically, how do I go about wiring this so I can use the horn vs. the siren for audible alarm? Thanks!

Answer
The wire you have drives a siren that the alarm came with.
If not avaialable then yes, you need a relay set up
as a negative to invert and use for the horn trigger.

Pin 87 to ground
pin 85 to const. 12 v. (fused at 10amps)
pin 86 to the alarm siren + output
30 to the horn wire in the truck
97a not used.

Have a good one.

ALARMTEK