Comment by eschneider

Interesting...1999 is probably a bit early for that Bosch to be running one of the usual ECU update protocols like UDS. It sounds like it's in the bootloader and looking for a valid executable. So the FW updater is likely in the bootloader.

If you can open it up and find the JTAG pads, it should be simple-ish to use a JTAG reader to dump the image and then you can figure out the update protocol from that. It's unlikely to be complicated.