Adding on the idea of a specific frequency; what if the car also sends a few ``honeypot'' signals at different frequencies, near the real frequency.

If the attack tries to amplify a given range, the honey pot signals will also be amplified, and the car can refuse to be opened.

You're assuming omnidirectional amplification. It's easy enough to amplify without leaking more signal back to the source than, say, a metal-faced wall would.