Copy the FLASH IC from a good one. Edit the MAC Address so the correct one is programmed into the bad chip (try FLASH address 000270080h). Reprogram the faulty FLASH IC. Connect to system and double-check MAC address is correct.
We've had LOTS of failing 1608L and 1616L phones, either flashing backlight with no test or completely dead. The above cures at least 1/2 the faulty ones that have come to us for repair.
Note that the MAC address may be stored in a different location for the 1608L, cannot remember off top of my head. Just do a search with the HEX editor for the last 4 digits of the MAC address (in HEX mode search).
Have fun.