Soldering to large metal case
I don't see the point in trying to solder to the case.  If you want to add a suppressor cap, put it between the leads.  You can then put another small cap on each lead to ground where it leaves the PCB.

You also don't know what metal the motor housing is made of.  Some metals are hard to solder to even if you can apply enough heat.  I doubt the housing is aluminum, but that is a good example.  Aluminum oxidizes so fast in air that you never get a chance to solder to the aluminum.  There is always a layer of aluminum oxide in the way.  Other metals are difficult too for various reasons.

As a aside, if you have a old soldering iron or old tip that is otherwise no good anymore, you can use a lot of flux, make a big solder blob, and then scratch the aluminum under the blob with the soldering iron.  The flux and solder blob won't let oxygen in there so the solder can actually wet the aluminum and make good contact.  I have used this trick to solder to aluminum foil a few times.