It also uses a small GPS puck antenna to figure out where it is and what to listen for. These usually work just fine if you place them up on the dashboard. They don't necessarily need to be mounted outside to work well.
Two years ago, I took a trip from Arkansas to northern Minnesota with my Home Patrol 1, with the GPS puck mounted on the dashboard. It worked great. Once I drove at least fifty miles before I noticed that the puck had fallen off of the dash and onto the passenger side floor. No matter, it still tracked where I was.