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. You'll have to use search engine to find adaptors for other makes of head unit
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 Mk5 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. You'll find it easier to install the unit if you remove the climate control unit and vents above the stereo. This unscrews (using Torx T20 driver) and unclips.
For an iPod, there is a piece of plastic in the roof of glovebox that can be pushed out so the iPod can sit in there.
I installed a double-din Pioneer AVIC-HD3BT sat-nav unit. This cost me £1500 but it was so worth it. There are so many features. Bluetooth, voice recognition, built-in harddrive, DVD player etc
Click
http://www.pioneer.co.uk/uk/products/10/20/241/AVIC-HD3BT/index.html for more details.
Sound quality has improved greatly. no distortion at higher volumes compared to OEM unit
Next thing is to add my sub (already pre-wired) and replace OEM speakers.
Any questions just email or message me
Matt