I have the same problem - Nuvi 680
I went out and bought another powercord w/ traffic antenna. Same problem. The unit itself seems to only use MSN software. Do I need to change my firmware to allow other traffic subscriptions? Also the above sequence "menu Tools, Settings, Traffic, Subscriptions" does not exist for my Nuvi 680.
The best I can do is "Tools - Navigation - Avoidances - then check the traffic box
Just stuck on how to get a TMC ID or which cable to buy exactly. Those things aren't cheap and I don't want to keep buying them using trial and error!
Any help is appreciated.
Edit: Found a GTM-10 10 digit code. It stays the same regardless which antenna I use though.