Viper 791XV horn trigger?

i take it you arent familair with relays? this is a simple task to change the + siren output on the 791XV to a - output using a relay.

if you arent familair, here you go (this is assuming your factory horn needs a negative trigger input);

pin 85 - hook this to the + siren output from 791XV

pin 86 - ground this terminal

pin 30 - this is your negative output that you will hook up to the factory horn

pin 87 - ground this terminal

yes, i forgot to add that in my post. good eye!