Not sure if you have seen this. https://www.golfmk7.com/forums/index.php?threads/diy-2016-gti-alternator-replacement.367315/ I noted that it has you remove the A/C compressor, but you do not remove the lines.
– Remove the A/C compressor from the bracket. Refer to ⇒
Heating and Air Conditioning...
That is bizaar. I would look at all the interlock switches. If DSG, that car is in park switch and brake switch. Not sure were the start relay is. Might be time to pull out the schematics for the start circuit.
There have been posts on using an adding a button at stick shift button location and wiring to your gutted garage door opener and stashing guts in area under stick shift. Easy and clean retrofit.
I purchased a VW mirror that had Homelink (programmable garage door button).
I had a similar experience until I performed the compressor adapt function described in the first few pages of the initial post.
Here it is: https://www.golfmk7.com/forums/index.php?threads/mk7-climatronic-upgrade-with-instructions-and-pictures.379148/page-10#post-7546622
I paid $200 for a replacment unit off of eBay. I had to use a VAS5054a to have the Component Protection removed and D link to install functional enablement codes. This is a post on a retrofit, but some of it applies wrt the component protection and FEC install...