Success with my 2017 Subaru Impreza, with the base head unit and the 2020.12.10.1057 firmware!
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Thanks to
@WPoly for the link to the 2020.12.10.1057 firmware, and to
@MrMister111 for the upgrade instructions. The 2020.12.10 firmware does work on my 2017 Subaru Impreza, whereas the previous version of the firmware did not.
It is disappointingly slow to connect. It takes around 30 seconds just for the head unit to connect to the CPLAYair dongle after I turn on the car (if the car has been sitting off for a while), and another ~15 seconds for the dongle to connect to my phone (iPhone 12 Pro) and for CarPlay to appear on the screen. (The head unit has always been pretty slow to connect, so that time isn't quite as bad as it seems, and I don't know how much of the slowness to attribute to the head unit vs the dongle. But it's certainly a lot slower than wired CarPlay in this car with this phone.) I might experiment some with the CPLAY2air settings, to see if a connection delay results in a faster overall experience.
The first time I tested after getting the dongle setup and then leaving the car off for a while, the car radio turned on immediately while I waited for the dongle to connect, which was disappointing. I had to go through my usual curse-at-it-and-mute-the-radio process, which I had hoped wouldn't be needed. (I really effing hate the Subaru head unit.) I've done three more tests, mostly to check the connection time, and in these tests the radio has remained off. This gives me some hope that maybe most of the time the radio will remain off. Time will tell.
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For some context and my installation experience: I ordered this CPLAY2air unit recently when they emailed me to let me know that Subaru (2017-2020) support had been added. It arrived a couple of days ago, and didn't work. I used their contact form to ask for help upgrading the firmware, and this morning(!) they replied with upgrade instructions (which matched those posted in this forum), and a link to "the latest firmware" (on a google drive).
The firmware they sent did not have any version-identifying name, but when I tried to upgrade the lights flashed (red and green for me, not red and blue as people have reported here) for about 12 seconds and then went back to solid red; I let it sit for several minutes and then tried it in the car but it didn't work. As the upgrade went so quickly, I suspect that it was actually the same version that was already on the dongle, and the upgrade didn't do anything. I did a crc on the firmware and compared it to ones linked in this forum, and it matched the 2020.11.10 version. I repeated the upgrade on the dongle just in case, but got identical results.
Then I tried the 2020.12.10.1057 firmware that WPoly had linked here — it still installed faster than I expected, but a lot slower than the previous "upgrades" (about 28 seconds?), and this time it worked. (Once the dongle UI showed up on the head unit screen, I had to tap the globe a couple of times to get the language to switch to English.)
So I'm happy to be up and running, and hopeful that it will be a big improvement in general. Thanks again for the helpful posts here!
