You have to coonect the trigger wire on the back of your H/U to the wire in the factory loom. Cant remember which wire colour but they did slighty vary through the models as i have found with the wiring diagram found on this site, a few were different to what my VT had when i done it.
If you have connected up all the other wires ie.(const 12v, ground, acc and all speakers) there should only be about 2 wires left in the loom which either will be the trigger for the antenna.