[Si Coe]

For the in-car option why do you need to cut all power? All you need if its running a brushed system is a relay switch connected to 1 of the motor wires. When the timer cuts out, the motor cuts out too, but you still have steering control as the car slows down.

Even better would be for the relay to slap a whopping resistor into the circuit, slowing the car to a crawl, and maybe lighting a big red LED on the roof. You can then get the person renting it to drive it back, saving the need for you to recover it at all.

But for reference the RC boats and trucks you sometimes see at amusement parks stop by cutting off the transmitter.