python 571xp horn honk wiring

i am attempting to wire the brown (-) wire from the alarm that is the horn honk output to activate the valve on my train horn..the vehicle is a 2001 Chevrolet Silverado with a Python 571xp w/ remote start already hooked up..when wired directly to the alarm it does activate the valve and honk the horn but when you use the toggle to activate the valve normally the car alarm siren goes off as well..the alarm install instructions advise to use a 5-pin relay if you have a (+) horn wire and when i wire it that way nothing happens and when i start the truck the alarm goes off..i am trying to simply get the horn to honk when i arm and dis-arm the alarm and don't want to short out the alarm or the valve for the horn would appreciate any ideas on making this project work..thanks"I'd rather be tried by twelve than carried by six"