John made some good points as to why a hardware solution may not be to your liking.

On the flip side side, a software solution can also have some draw backs. I am coming from an industrial point of view so maybe something non-critical as a RC car could be looked at differently...

To go from manual to auto with software at least one ASM interrupt is needed, depending on the application you may want more from different sources. In your case on should be enough. If the operator can not see the car then switch to auto. Using software the code can be cumbersome. Say you are running in manual. Are you still keeping track of the sensors? That can get to be a neat trick, code space and all. If you do a hardware switch, the auto part can keep monitoring the inputs just as it would if it was in control. This way, when auto does take over it is near seamless.

Good and bad both ways.

If you do go with the hardware solution and the RC part of the car (manual) is still intact then you are halfway there. Find were the inputs to the MOSFET bank are or the motor controller and that is where the DPDT relay will go. Being the control signal is the only thing that is switching the only surge you will have to worry about is from the relay coil. A flyback diode will take care of that.

But either way you go, keep us posted. I have some young ones I am also teaching.

BTW.... I only use windows when I have to...