i dont really understand what u mean when u say the alarm is sensitive... all 12v sources in the car should give the same current as the battery, i wouldnt pull current as highlighted from a headlight or other source as u may find it affect the running of that item.
if it has to goto a non battery source lengthen the wire and runit into the car, it shouldnbt have to be to long and the benefit of internal wiring means its harder for the theif to cut it and gain entry to the car, with an underbonnet system the can force the bonnet and cut the feeds then break in and theyr away..