Got my APRS TNC today and tried it in the car. It needs to be out in the open to get a good GPS signal. Made a cable with 9 pin D-type connector to a 6 pin mini DIN, and powered it from a small regulated power supply (one with selectable 1.5 - 12v and is built into a lighter plug). Had a drive towards Ulverston to see whether I was getting received by anyone (including my own station at home). On the way there I had the TNC in a bad place for GPS signals so it didn't work, but it worked OK lying on the passenger seat.
As well as receiving my own packets I was received by G7HEJ and G1EUH.
I wasn't able to change any settings in terminal mode because I didn't have the right connectors on my serial cable but when I've got the right cable I'll make sure everything is configured properly. It had already been set up for mobile operation with my callsign when it was sent to me.