Horn

I am trying to hook up my horn to sound whenever I hit the lock button on my remote.  Problem is my starter/keyless remote unit has no output for it, so I wired up a relay to take the positive output from the door lock wire and output a negative to the horn.  I get the horn to sound but its too loud.  I introduced a trimmer into the mix thinking the added resistance would help quite the horn, No luck, either it sounds or it doesn't  depending what resistance I have it set to.  Anyone know how to get this to work?  I only want the "chirp" when the doors lock.  Any help would be appreciated.

It is a remote starter/keyless entry, audiovox aa-rs5cs. I have it installed on a 2000 Chevy Blazer (no factory alarm).  Like I said, I can get it to sound but its too loud. I just found another wire on the unit, says its a 300mA pulsed ground output, think I should try and connect this to my horn for the desired "chirp" or do I need to take another route?