I've not heard of anyone running a VW box in a SEAT, but that's not to say it isn't possible.
Unfortunately I suspect it's very unlikely anyone here will be able to give you much information other than giving you the suggestions I did, so seat owners clubs are really your only best shot at getting the information you want.
As I hinted, it's possible they used a VW/Audi headunit that was compatible with a PhatBox?