If it's an aluminum casing, you will never be able to get the solder to stick. Best thing you could do then is to fasten a steel band-clamp around the casing, after soldering a wire to the band-clamp. Make sure the area under the clamp is clean and perhaps apply a dab of lubricant to keep it clean. The clamp will push the lubricant aside to create a metal-to-metal contact as the band is tightened. Don't over-tighten it; snug so the band doesn't rotate is all you need.
It is just barely possible that you are not heating the casing hot enough (because your soldering iron is too low wattage), or you are not using enough flux to "wet" the surface of the casing, which probably acts as a good heat sink... all that is assuming the casing IS solderable. See previous comment on aluminum. If the casing is solderable, you still run the risk of damaging the internal components of the motor by heating it while applying solder. I would try the band clamp method first.