The wiper and window motors we use (denso) do not have the torque to steer the vehicle. In addition, they are not built for continuos cycle and at high torques will trip out their internal thermal breaker. They don't have the rotational speed you would need to gear them down far enough and still have responsive steering. They also aren't terribly robust - they strip their worm gear out fairly easily.
Perhaps you could use one of our inexpensive "cim" motors which are quite powerful, durable, and inexpensive, and run them through a reduction transmission. I'd think that a ratio of 48:1 or a bit higher would be a good mix of speed and torque.
You could do that with off-the-shelf transmissions here:
http://www.andymark.biz/am-0141.html (3 stacker boxes)
Or here:
http://banebots.com/pc/P80K-nnnn-0005/P80K-444-0005
I would run an auto-resetting 40 amp fuse through the banebots transmission. The cim has 103 oz-inches @ 40 amps. That's 0.536 lb/ft of torque, and through the transmission will be 34 ft/lbs. (Someone can check my math if they like). That's a safety factor of 2 against the rating of the banebots gearbox. You could drive the steering with 35 chain from mcmaster, and simply have a foot control that turns the current on and off, left and right.