What's the best way to get traffic information on my new-to-me Zumo 550 when used IN MY CAR with the speaker/cradle? It came with a GXM30 antenna, but no subscriptions. I think I could get a GTM21 FM antenna, and lifetime traffic subscription. That would run about $100. That's pretty steep! Is there a better way? Note that I would only need the traffic feature when using the Zumo 550 in my car. Thanks.