I bought a MagSafe charger mount in anticipation of the
wireless CarPlay rollout. Since the upgrade, I've been struggling to get the phone to charge - the best case seemed to be that it maintained it's charge level while in the car. I think I've figure out what the issue is...
I had the MagSafe charger connected to the data-enabled USB port, which I had previously used. It is limited to 5V - and matched with the MagSafe charger, which could draw a maximum of just over 1A (designed for a 12V supply), resulted in a 5W maximum draw. Given whatever inefficiency/overhead the wireless charger had, it seemed that it could only deliver net power roughly equal to whatever the phone needed to persistently maintain the Wi-Fi connection, etc. required for
CarPlay.
The other USB ports in the car support 9V, and I've seen them hit around 25W when charger a laptop via USB-C. When matched with a new qi2 charger at 9V, it can deliver close to 15W of power to the phone. So, the lesson here is to avoid the data port for charging and make sure any charger you use supports 9V.