I've been trying the APRS tracker in the car some more and find that it must be fitted the right way up and on top of the dashboard for it to work (like any satnav). If there is no GPS signal it still transmits about once every 3 minutes but with invalid GPS co-ordinates. Going along the A590, I can receive my own mobile signal at home until I get into Ulverston (about 6 miles away). That's with about 10 watts into a 1/4 wave magmount on the car. On high ground I'm getting received by some other stations.
This picture shows an installation which didn't work, the tracker box has to be lying flat. By the clock is a better position, sitting on top of the air vent cradle.
I've got the FT8900 fixed to the ashtray using a binder clip taped to its head unit. The rest of the 8900 is fixed inside the glovebox with velcro tape. Everything runs from the lighter socket so I never use the full 50 watts. The white box inside the glovebox is the connections for the power and audio to/from the TNC.