The AV inputs and source switching features are on the TV module. With digital switchover an analogue TV module should be about £60 and the TV retrofit cable is about £35 new.

The only other option (a bodge that requires some serious wiring and PCB making skills) would be to throw the RGB line from the nav to a freeview tuner. You'd need one that could do HVGA and Sync on green native resolution to match the monitor. You would not be able to view the vehicle menus in that configuration since they are generated by the nav drive.