[Android Head-Units] unimatrix099:
Oh, had the same issue with exactly this unit when installed the custom ROM - the Malaysk rom (looks like the Audi unit PX6-STM32, I'll attach a picture with the exact unit name). It is not easy to recover the unit, but it can be done... I'll add here a quick tutorial (it's been more than 6 months, just writing from my memory, double check the steps), but the main idea is that you to enter Loader mode to dump a new firmware on the unit (not an update zip file, a complete dump, I made a backup dump before installing the custom ROM, that's why I still have the unit work... always backup... )

1. Make a custom OTG USB cable - the OTG USB D+/D- pins are in the 6 pin connector (in the manual are marked as NC - not connected, but they are :) ) - there is an image attached with the pinout. I used the 6 pin USB cable provided with the unit, just moved the pins to the D+/D-. Also you will need an USB A to USB A cable. The problem is that the connection is very 'weak' use a very short cable and only USB 2.0 devices, I've tried a lot of devices (laptops, PC, raspberry) the only one that I could make it work was with an Orange PI PC development board, maybe it was my cable, maybe it can work better with a cable from Aliexpress that connects directly to the OTG port (small 4 pin white port)

2. To enter the Loader mode you will need to connect 2 pins when powering the unit (image with the location attached) - yes you need to open the device :(

3. Use a loader Realtek tool to dump the firmware, in my case, I used the Linux tool rkdeveloptool -> https://github.com/rockchip-linux/rkdeveloptool (but I think there are also Windows versions of the tools, can't remember now the names)

With rkdeveloptool I used the following commands (in this order):

"rkdeveloptool ld" - list device to see if the device is in loader mode

"rkdeveloptool rd 3" - reset device in maskrom mode (this is not documented, it was just a random comment in github)

"rkdeveloptool db RKXXLoader.bin" - load a new loader program

"rkdeveloptool wl 0x8000 kernel.img" - write each partition file with the starting point in the flash memory, here the command is to write the kernel.img file to the 0x8000 location in the flash memory, the partition information is in the parameter file in the firmware. From the file: "0x00004000@0x00000000(EFI_part),0x00002000@0x00004000(uboot),0x00002000@0x00006000(trust)" - the first part is the size of the partition and after the @ is the location, so EFI_part is at 0x0 and size is 0x4000, uboot partition is at 0x4000 and size is 0x2000 and so on...

After all partitions are written to flash, restart the device and it should work.

I had issues with USB communication, sometimes it stopped, sometimes the data written on flash was compromised because of the USB connection, I had to write some partitions multiple times until it was correct... I will get back with another post with a possible solution.

Maybe resetting to maskrom mode is not needed and use the loader that is already on the device (so maybe you can skip the "rkdeveloptool rd 3" and 'rkdeveloptool db RKXXLoader.bin' commands.

Firmware dump link - https://drive.google.com/file/d/1po-Vkbtom-M4ELWNkXeYuiyyCrhm33-2/view

Good luck with restoring the device :)

Update: I forgot to say, this unit is the version with DSP, so the image is for units with DSP, maybe you can make it boot and then update to the proper firmware