Having to borrow a car with a metal roof to test the system is a pain.  The antenna spacing for VHF is about 18".   My cars have sunroofs - metal tops with big glass holes.   There is no way to attach the four magnet mount monopoles with the required spacing because of the sunroof location.  I can magnetically mount to the roof with a trestle over the sunroof but that is a problem for the monopoles - no uniform ground plane.  That can be solved by going with a dipole array.  The height of the array would be about 36" plus the trestle height.  Rubber duckie's for the elements might be a tradeoff.

A dipole array can be a single unit so a positioning sensor can be built into it.  The system is needs be calibrated once and shouldn't need recalibration between installations and removals.  

Here's a stationary array - just four antennas should be sufficient: