Relay, not replay attack. For vehicles that unlock and push to start based on the proximity of the key, basically all you need to do is use a pair of software defined radios to trick the car into thinking that the key is there. Hence a relay attack, where one attacker is located near the key and the other is located near the vehicle, and the two attackers pass the components of the challenge/response to the vehicle via the channel between the two attackers.
but now we're seeing a rash of thefts of keyless fob vehicles with a relay attack making me glad my tacoma needs a key