US cars with RTTI use a different method as daflake said. It's provided by clear channel and has nothing to do with radio stations (like TMC uses in Europe)...

If I recall correctly it's actually done through Satellite radio? Have a look: "Unlike other automobile manufacturers that charge a recurring monthly fee for RTTI, or require customers to pay for an additional satellite radio service, BMW has included RTTI as part of the cost of the navigation system for select 2007 BMW models. "

So basically I think BMW is installs the satellite radio unit in when you order Navigation - it just isn't activated unless you pay for the Satellite radio option - despite being active the entire time to enable RTTI.