Hi Darren,
Do you know where the firmware is located on the actual RNS unit?
is it stored on the harddrive or is it on an eeprom on the motherboard?
(I'm not tooo familiar with car electronics but a bit of a general electronics specialist, and i know for a fact that if the firmware is stored on an eeprom/memory chip on the motherboard, direct connection to this to flash a clean/safe dump to this chip will restore the unit. not necessary to replace the working motherboard unless of course is physically damaged. i've recovered and cleared codes from 'no-good' ABS and airbag units this method, just read the eeprom, alter the dump to clear the data and re-flash and thats that!)
cheers