Thanks to the original poster for this. It inspired me to give it a go, and I have just finished the job an hour ago.
Picked up the Nav and arm on Ebay for £125 as new. Got the harness on Ebay brand new from a UK supplier delivered for £59. So the whole job has cost £184
I'll order the cover plate in due course from my local Seat dealer as the parts are the same for VW and Seat (and Skoda)
Some pointers, and other things I picked up and noticed.
The original posters photos are very accurate. I just made a cardboard template by holding it agains the base of the Arm bracket which fixes to the dash. Take the imprint from the inner line, and don't make the hole larger that the outer outline of the Arm or it will be noticeable when you put the arm in place.
I trusted the template outline on the dashboard the OP posted and drew mine round the exact same place. The dashboard plastic is actually very soft, and I completed the job with a small wood drill bit, and a blade. I drilled numerous holes in a line inside the template and then cut it out with the sharp blade. The plastic is easily cut. This first opening was intentionally too small, but you can now see what you need to inside the dash, and I just kept slicing it larger and larger with the razor blade until I could get the wiring harness and connector into place. A scalpel type tool would be perfect for this.
Then I had to enlarge it just a bit more to get a good fit for the Satnav arm. I was opening the hole mm by mm, as I did not want the opening to end up too big. All went perfectly.
Note, there is absolutely no need to buy the metal bracket for screwing the harness connector to the dash. For a start its about £22 which is ridiculous. The head of the wiring harness seats quite firmly into the newly opened dash hole, I simply run hot glue around the bracket and pushed its seat in the hole. Its going nowhere, but its easy to remove again if I have too, such is the nature of hot glue. Once the bracket arm was clicked on top, it was like a factory fit, good and tight and none of my handiwork visible.
The wiring loom and radio wiring loom can be difficult to seat properly when placing the radio back in its housing. It tends to kink and stop the radio unit going fully into the dash. to prevent this, follow these instructions.
With the radio out, look in to the cavity. At the back there are two square holes about 3 inches square on the left and right at the very back. The wiring to the radio already comes out of the left cavity. Ensure when you run the loom from the maps and more connector inside the dash, you run it down the back of the cavity, feed it behind the heater control panel and exit through the left cavity, (same side as the radio wiring). Tape or otherwise fix the new maps and more loom to the existing radio loom, and push them as far as possible back thought the left cavity at the back of the radio slot. Hopefully as you push the radio back in to place, the loom will stay straight and push through the hole at the back of the radio slot and you'll be ok.
All and all, I am delighted. Done this mod for my daughter and she is delighted with all the extra stuff she can now do.
I really want to have this fitted. Thank you for this. All I need is the harness wiring, the bracket and the cap.
Can I buy it second hand from ones who don't use that kind of Satnav or who sold their cars?