I have a nüvi 250W (not a nüvi 250WT!) and I have a GTM25 traffic receiver for it. When connecting it, I get the message that this accessory is not supported (screenshot not taken from actual unit!):

ctgq

Is it possible to enable TMC in this unit? I suspect I have to flash the proper firmware for this. If that is the case, can someone point me in the right direction where to find the correct firmware?