to install an aftermarket headunit you need 4 adaptors:
1) an adaptor to go from car to the unit itself.
http://www.incarexpress.co.uk/view_product.php?partno=CARPI221
this adaptor is for Pioneer units and will let you use your steering wheel controls.
If the headunit is not steering wheel control compatible then click below:
http://www.incarexpress.co.uk/view_product.php?partno=PC2754VW
2) the SEAT sat-nav fascia adaptor.
Go to your SEAT dealer and ask for part no. 1P2858071A. You will need to give your reg no. to get the correct colour trim. this part cost me £8.45.
3) do a search on eBay for a VW Golf fascia. (approx £9-15) This will fit in the SEAT sat-nav fascia adaptor and then you can put in your aftermarket headunit.
If installing a double-din unit then you will need to cut out the middle bar in the VW Golf fascia adaptor.
4) a Fakra aerial adaptor
http://www.incarexpress.co.uk/view_product.php?partno=CT27AA14
Also there is loads of space for wires, iPod connectors etc behind climate control unit.
I installed a double-din Pioneer sat-nav unit, cost me £1500
but it's was so worth it. I can now see how many g's I'm pulling whilst accelerating and has a Knightrider style speedo (i.e goes thro all the numbers) also sound quality has improved greatly. Next thing is to add my sub (already pre-wired) and replace OEM speakers.
Any questions just message me
Matt