Your large antenna will not fit inside a car.

 

Your large antenna will not fit inside a car.

Certainly that is true, if it is not modified to be smaller, which is something that Ham Radio operators have been doing for many years. Coiling the two sides of a dipole can reduce the length quite a bit, at the expense of needing more wire. And there is also the option of making the ends quarter wavelengths, at which point they do fit.
Of course the simplest way is still adding a connection to the vehicles radio antenna connection. Simple, direct, and quite possibly cheap as well.