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You may be able to fix the secondary air pump noise by drilling out the rivets and replacing them with bolts. Pretty simple job. I've done it on both my Octavias. I think there's a step-by-step guide on the UK MKIVs site.

You can put your car reg number into a car parts website and look for door loom or window regulator. No offence meant, but if you can't find the parts online, do you have the knowledge to diagnose which part is faulty, and the skills to install a replacement? Maybe have somebody more knowledgeable look at it so you don't end up buying parts you don't need. Then have a go at doing the work yourself.

The door wiring loom looks a bit butchered. You might need to replace that. The part that the wiring plugs into is the window regulator that raises and lowers the glass. That has nothing to do with mirrors, so most likely the wires going to the mirrors are broken too. Usually they break where the wiring loom goes from the door to the pillar.

Here's a link to a picture of the cable. Might help you find it on the engine. https://a.allegroimg.com/s650/11e4a7/da124d5d4a3f9ae4238783e25698/Audi-KABEL-GRD-8L0971371

Still available?

Most likely the broken wires are in the gap between the door and the pillar, where they constantly bend. The door loom can be disonnected at the pillar by rotating the connector on the pillar. Then you just take off the door card and take the loom out. Or take the whole door so you have a spare lock, handle, window regulator and switch.

€636 in Ireland. Has been for at least 10 years now.

Check if a heat shield that should be welded to the exhaust has come loose. Happened on my VRS and made the same rattle as you describe. As far as I recall it was about half way along the exhaust.

I think there might be two types of clip, depending on position on the door. Search for part number 1U0 867 260. I've sourced these on skoda-parts.com previously.

Sounds like the immobiliser is shutting it down. May be a problem with the transponder in the key, in which case your spare key should work. Issue might be also be the immobiliser reader at the igintion switch, or wiring from there to the instrument cluster. Search this forum for "immobiliser". The problem has come up previously.