How'd those Radio Shack sirens work out? That is a heck of alot cheaper than the HD! I might have to go if you folks tell me good things.got the siren and am getting ready to hook it up to my daughters sportster. a bit cheaper than the HD siren/chirper. unlike the HD siren, it doesn't have a battery backup but will still work with the original security system.
Without knowing the current draw and where you are wiring it up, I would look very carefully at the siren specs and make sure you do not damage the HD ECM/Alarm module. To be safe, you may want to use a relay to minimize current draw to the relay coil (20mA or so) and the dry contacts to switch fused power to the aftermarket siren/horn, to be on the safe side, JMHO... :s
An ECM replacement would VERY expensive to replace, especially if damaged by improperly installed $10 aftermarket part.