I just wanted to update on my situation above because I have no doubt someone will have this issue and will be as desperate as I was. I managed to fix it!
The culprit is the RT6's MicroSD card. Inside the RT6 is an 8GB MicroSD card that stores the firmware. As with everything, these can become corrupt over time and fail. This causes a myriad of issues such as the RT6 crashing and rebooting, the RT6 getting stuck in a boot-loop, the RT6 saying "Checking in process" every single time you start it, the RT6 failing to upgrade firmware as mine did, etc.
The fix is simple - it needs a new MicroSD card. You first need to remove the RT6 unit from the car. I used the following YouTube video of a Peugeot 308 that has the same dashboard layout as my Peugeot RCZ (it has English captions you can enable):
Once you have the RT6 in your hands, you can follow this guide to change its MicroSD card:
How to Replace the Internal MicroSD Card - RT6 - French Car Forum
I didn't do the battery disconnection procedure. I simply left the car off with the battery connected the whole time. Anyway, the RT6 does not support a MicroSD card larger than 16GB. I found a Sandisk Class 10 (high-speed) 16GB MicroSD card for €5 and went with it. You do not need to format the MicroSD card first. Just take it out of the original box and put it into the RT6. The RT6 will format it once you install firmware on it.
Now, since the MicroSD card is blank, the RT6 will start with a blank screen and the disc drive will make a noise every 30 seconds or so, as if seeking a disc. You will need to use a USB stick to flash the firmware on to the RT6. You can download the RT6 2.86 firmware from the links posted above by
tianorth.
The USB stick you use must also be 16GB or smaller. Format it on a computer with the FAT32 file system. Once blank, copy over the contents of the RT6 2.86 firmware update on to it. Then, open the Windows Command Prompt in administrator mode. Note which drive letter in Windows is your USB stick (say it's E drive), then type:
del "E:\System Volume Information"
This is a hidden folder Windows creates on newly-formatted USB sticks and its presence makes the RT6 firmware update fail. As soon as you use the above command to delete the folder, unplug the USB stick from your computer and do not plug it back in (as it will re-create this folder when you plug it back in).
Insert the prepared USB stick into your car's USB port and switch on the car's engine. The RT6 will pop up and say "CRC checking media" as it reads the USB stick you plugged in. It will flash the firmware automatically. After about 15 minutes (with a new, high-speed MicroSD card, this is what it takes), you'll be sorted!
An issue you may have after you do all this, is lack of audio output from any source (Radio, AUX, Bluetooth, etc). At least this was the case for me with the JBL addon for my RCZ. The button on your steering wheel to change audio source may also not work anymore. To fix this, you need to now use the
Deactivation of Activation Code files posted by
tianorth above. You need to repeat the process above to prepare a USB stick with this. Namely, formatting the USB stick (16GB or smaller) as FAT32, copying the files over and then deleting the
System Volume Information folder that Windows generates, using the Windows Command Prompt.
Once your USB stick is prepared, insert it into the car while the car is on and the screen will begin asking you a series of
Yes or
No questions. Answer
No to everything until you are asked about
Audio Mode. Here, for me it was currently set to
3 and I was asked if I wanted to change it to
4. For the JBL addon on your RCZ, you need to set it to
2. Just select
Yes multiple times until it sets to
2 (it will cycle through values). Once set to
2, select
No to continue. Again, answer
No to everything again until you are asked about
VMF/VCCF. Here, make sure it is set to
0. Mine was set to
1 and it asked me if I wanted to change it to
0, which I answered
Yes to. Once set to
0, keep answering
No until you get to the question on
Maps DRM. If this is currently set to
1, answer
Yes to change it to
0 and then proceed. At the end it will ask if you want to reboot. Definitely answer
Yes here, as the reboot is needed to apply the settings. When the screen turns off to reboot, remove the USB stick from the car.
When your screen returns, you'll have your JBL audio as well as the steering wheel's button for audio source working again. Having set
Maps DRM to
0, now allows you to install the maps and map warnings as per instructions in this thread. You can leave this setting as it is as well (no need to change it back to
1).
For reference, I found what each of the
Audio Mode values represents:
0 = Normal (internal amplifier)
1 = Hi-Fi (external analogue amplifier)
2 = Hi-Fi CAN (external digital amplifier)
3 = Arkamys (internal amplifier with Arkamys DSP audio)
4 = Arkamys + Hi-Fi (external analogue amplifier with Arkamys DSP audio)
I tried
3 (the default set when I fixed my RT6) and
1. Neither worked with my JBL addon.
2 is the correct setting it seems.
Hopefully the above helps someone.