Convert RF to DC, specific frequency with extremely low current circuit
the other side must be extremely low current because it will ALWAYS
  listen and will be powered by a much much smaller battery (compared to
  the car)


Wireless remote receivers are available with current consumption as low as 70uA, but this is still much for the typical battery that fits in a keychain remote. Here are a few possible solutions:-


Put the receiver in standby or power down (drawing 1uA or less) and wake up it up with a timer just long enough to check for a valid signal. This could cut the average current consumption to 1% or less, but increases latency to the period of timer.
Use an ultra-low power 'wake-up' rf detector that receives any signals close to the frequency of your transmitter. This would wake up the micro, which then turns on the main receiver to receive the actual signal (if present). To avoid triggering on spurious signals or noise the wake-up receiver should have relatively low sensitivity, but high enough to reliably detect the signal from your car at short range. Here's an example that uses a Schottky diode detector and only draws 400nA:-

Use a rechargeable battery that is topped up regularly so it doesn't have to last for more than a few days. Perhaps have a wireless charging station that you hang your keychain on, or some kind of energy harvesting similar to a self-winding watch.
Put a button on the keychain which you press to enable it before use. Alternatively make the button actually unlock the car - then you don't need all the other stuff!