Here are the 2m and 70cm SWR plots for the Nagoya NL-77B dual bander on the new car. This is with a small Sirio magmount towards the back of the car nearside. There is less than a metre between the dual bander and the broadcast antenna.
For some reason the Nagoya doesn't give a very good SWR on 145 MHz, this was the same on the old car and with a different magmount. It's under 2:1 though.
At UHF though, it's very good if a bit high in frequency, there aren't the usual reflections on the coax, this might be to do with the magmount and coax being quite new.