It doesn't get much easier than this:
Just a beam running through the car attached to both bumpers.
There's a hole in the beam; the handle of a DPDT switch goes through the hole; the body of the switch is mounted fixed to the car body.
When the front bumper strikes something, the bumper beam flips the switch to the other direction, reversing the polarity of the motor.
No gates, flip-flops, relays, or anything else is needed.
the problem is that i might not be able to find the component ....
for than that, is there anymore simpler one?