Grumpy_Mike:
Just solder it up, that plastic is quite tough.
Personally, I would not bet on that.
Ensuring the tabs are clean is perhaps the first most important thing - a gentle polish with steel wool whilst holding the motor so that the tabs point down and any debris from the steel wool falls downward and away from the motor as you do this. Actually, that is probably not such a good idea to do with a motor as its magnets will tend to attract the steel wool. A cleaning compound such as "Jif" may be more appropriate, applied in a "pinch" on a damp cellulose sponge, also whilst holding the motor so that the tabs point down and any moisture drops away, then completely wiped off with a rinsed sponge.
The skill in soldering is to apply the iron to the tab and touch the solder not to the iron and not just to the "work", but to "wedge" the tip of the solder to the very angle where the iron is actually contacting the work so that the solder as it melts, immediately conducts the heat from iron to work and flows in the capillary between.
And - you are using too heavy a gauge of solder; finer gauges release more flux.