Those alarms have a diode built in. Be sure your polarity is correct. Best i can remember i used to install the brand of alarm you mentioned next to the horn and ran a jumper from horn negative to b/u negative post then picked up the positive from reverse directional switch leg. These were a 97 or 97 db. Find the one that has the lowest operational amp draw. The oem alarms are a 3 wire set up and a pain to get at as you have to remove the battery. With with it mounted underneath floor pan, it is more protected and less likely of the operators pulling the wires to "hush" it.