i think '5N0 035 342 C' should work, but this isn't for USB, it's only for IPOD !!
i might give it a try in the future.
It's for USB memory sticks or iPODs with the correct lead.
6J0 035 342 C is the exact unit from a USB enabled Ibiza. I know because I took the picture:

The ones that have a B at the end of the p/n look different to the one in my car but may work the same apart from the shape being different.
The USB unit is supposed to be mounted above the cup holders like in the pictures.
One question, does a non USB Ibiza's head unit have a 'MEDIA' button to switch to listening to USB/iPOD?