If the power antenna is factory fitted i'd of thought it would just be picking up a feed through the blue wire on your ISO block, check to see if the blue wire on the HU ISO correlates to the cars wiring harness and leads up to the antenna. If not find out where the power antenna wires up to normally, if it goes to the ISO block on the cars wiring loom and it's a different pin you may (very unlikely this is the case and a bit random) need an ISO adapter. What car is it, as this will help no end!?

If not i'd just splice into the blue wire yes, but think it through first and try and replicate the factory wiring positions as best as possible