tom/tasdrouille's idea sounds good.
What you could do is put those one sliding panels on a track with grooves, and the other fixed to the track. Then drive the sliding panel with a worm gear of sorts. Perhaps add in some basic circuitry that turns off the motor when it reaches the maximum slide area.
The worm gear would resist slippages during high speeds, and provide tons of torque.
edit: heres a concept design I sketched out:
http://bob64.binuweb.com/misc/prius/...gnconcept.html