I've just done this on my
Mk2 Leon (but probably not different for the
Ibiza).
Open your boot, then pull the head lining down from inside the rubber seal, around the back edge of the roof. You can now push your hand in and carefully pull the lining down from the roof (it might also be lightly bonded + clips too).
There are 2 leads coming from the Antenna base - one being a power feed, plus the other being signal. Suspect you have a short circuit or issue with the power lead connector.
EDIT: try not to FOLD the headlining down - keep it as straight as possible - , or it could create a crease, when you fit it back up again!