The car already has 2 x TV antennas and 2 x TV antenna amplifiers (boosters)

The standard system is extremely well designed and engineered and works particularly well. You won't be able to do much at all to improve it.

Apart from mounting a steerable yagi antenna on a 10m pole on your vehicle roof :-)

If you suffer poor reception, it is due to low signal levels in your area. Drive to a different place!