I was thinking a "tunnel" type thing, but now I see that is not going to work. Maybe the light idea is not the way but still on that thought.

Have you seen a "line follower" setup? An IR sensor follows a black line...
What about having the floor in the maze black and an IR looking down on the front of the car?

Or...

A horizontal IR beam at the beginning of the maze and one at the end. Sees IR once the car is on auto, sees IR the second time and it is back on manual.