At 77GHz the size of a car is more than 1000 wavelength, how to simulate an antenna in the environment? 

Do you know that it is possible to split the whole structure - meaning antenna, radome and a car - into separate sub-domains and simulate each domain using a different solver?

This is a Hybrid Solver approach, and it is very efficient when it comes to electrically large structures with a lot of metal and empty spaces.

For example, one can simulate the antenna array only once, and then use the field sources to do tolerance analysis for dielectric thickness of the radome. Later, the optimized assembly can be placed on a car's bumper and simulated with the environment.