You probably could tie it diectly to the +pin of the PWR_SW header on the mobo however I would still use a relay. You can use a small 12V 2A relay, they are much smaller than the typical 30A relays. Connect one input of the relay coil to 12V from the battery and connect the other end of the coil to the output of alarm. Connect the 'normally open' pin of the relay to ground and connect the 'wiper' pin of the relay to the +pin of the mobo PWR_SW. Keep the 'normally closed' relay pin floating. FYI, the Carnetix and possibly the OPUS have a dedicated input to connect a car alarm output to for remote starting the carputer.