viper 5000 accessory line addition

I have a 2006 Scion tC with a Viper 5000 alarm installed. Vehicle Diagram:https://www.the12volt.com/installbay/forum_posts.asp~TID~59369 I am going to install a train horn in a couple of weeks. I want two things: to tie the train horn sound circuit into the stock horn with a switch so I can either sound the stock horn, or both horns. The second thing I want is to wire the train horn sound circuit into an accessory port of my alarm so I can remotely activate it if want to. A few details: The scion stock horn is a (-) triggered circuit as described above. The accessory port on the viper outputs a 200 mA (-) current. Will this work: Using a SPDT Relay (no diode in it), I would: Pole 85= Put a 12v Constant fused at 5 Amps Pole 86= Put two wires: Tap a wire from the stock horn circuit to here with a switch inbetween to control activity. Second wire: Put the alarms 200 mA (-) accesory line here. Pole 87=Line to Solenoid of train horn. Pole 87a= Not used, insulate from being exposed. Pole 30=12v Constant fused at 5A If that doesnt work, please give me some constructive criticism. BTW, if you are wondering why I want a train horn, Ive been hit 3 times this past year in my car by SUVs who didnt see or hear me and merged into my car...

Bump! Any thoughts?

isolate the wires going to 86 with diodes. you dont want them to backfeed on one another.

Alright thanks! Any other thoughts or comments?