It's my understand that in the US, the manufacturer can only void the warranty for parts of the car. Like, this retrofit can't trigger them to void the warranty on the brakes. But the can absolutely void the warranty on the stuff I futzed around with, which would be the rear A/C louvers and the USB ports, probably the center console too. "Around that area of the car" is a fair way to quickly convey this, I think.
Anyway, the back part fit beautifully, using the adapter shipped from Germany from the eBay store here
. I now have close to 50W in the back, which is going to be really helpful on road trips. It took maybe 5 minutes, and was about as difficult as replacing a hubcap.
The adapters I got look well-made. I do a fair amount of hobbyist electronics work and this is at least as good as anything I could have put together. I didn't want to buy all the parts because I only needed two, but this actually looks like something I could have made myself with just a pair of pliers, and I feel a little foolish now having shipped this all the way across the Atlantic ocean. So to any future thread-readers, you really ought to look into how the two connectors crimp (I think they're linked earlier in this thread) and consider making your own connector. You could maybe even ship the leftover parts off to the next person in this thread who wants to do the upgrade, so you don't have to deal with a bunch of extra connectors you will never use for anything ever.
I decided not to do the front retrofit until my family all has phones that can charge wirelessly. There does not appear to be a simple way to evenly distribute the 24W on the left port to two devices: your best bet is to plug one phone directly into the USB C port, and continue using a USB 3 hub on the right port, which means "slow" charging for the other phone. That's not enough of an improvement for me to bother right now.
I encourage anybody contemplating it to at least do the back ports. It's a simple operation, the part is fairly inexpensive, and the result is a substantial improvement.