Here's a really rough sketchup of what I was talking about in my first post:

The center part is the switch pad.  The round things are basic button-type switches.  The arrow part pointing down would have to be attached to the steering / tie rod and the large upper bracket would be stationary on the axle.  On each side is a plunger / paddle with sliding shafts.  On each shaft would be a spring.  (I didn't feel like modeling up the spring, you'll have to use your imagination there.)

In the picture the tie rod has moved right, pushing the right switch into the sprung plunger / pad and thereby activating the switch.  The switch would be wired through the five pin relay so that while you were holding the right switch in the cab nothing would be happening at the switch on the axle other than being depressed.  As soon as you let off the switch in the cab power would be applied to the switch at the axle which would be wired to the left side on the solenoid valve, thereby activating the valve and turning the steering left.  There would be just enough space between plungers / paddles so that in the center neither switch was depressed.  Therefore, the system would stop acting once the switch on the right in this pic stopped being pushed by the sprung plunger/ paddle.  It works the same way in both directions.  This setup does not require a computer / PLC / logic controller and by mounting the stationary top bracket on a threaded rod on the axle housing with nuts on both sides exact center could be fine-tuned as things bent / settled over time.