There are other older threads on the door motor/winches. I suggest searching the forums and you'll get some good info. We put a winch spool on the van door motor assembly (maybe 1 1/2 diameter in the center and 3" for the end plates. You can turn it relatively easily by hand, so we are putting a positive hold mechanism (like a cleat or an ascender for rock climbing) onto the cable to keep it from unreeling once we lift.
Worm drives can definitely be back-driven if the lead angle is too great. There is some info about this at bostongear.com at
http://www.bostongear.com/products/enclosed/faqs.html (and possibly in the "gearology" tutorial, too) and a mention about back drive efficiency (Eb) somewhere at
http://www.roton.com/worms.jsp. I saw this from a random search, so don't take it as hard fact: "A rule of thumb for the worm drive is they wilt not back drive above a 20:1 ratio. If vibrations occur, however, the ratio will need to be higher. Self-locking in the gear pass depends on the coefficient of friction between the worm and the worm wheel, which depends on the choice of materials and the lead angle of the worm, which is a function of the desired ratio."