The window motors may have an issue.... Most have some provision for not breaking fingers that may be in the way. While that could be yet another function of some sort of local controller, it may also be an integral overload in the motor. That, if present, could be a limit on the motor power/torque.
As for the switch, it does appear labeled as a DPDT, ignoring the "don't use" note. As to what switch brand it really is, and whether it is 21A or not, no clue. There are two different switches shown, one for the wiring, and a different one for all the other views. I cannot read (or in some cases find) any manufacturer data or any ratings. The basic Carling switch is rated 15A at 24VDC, which should be enough.
The middle six terminals should be the ones, per the diagram (which you can check with a volt-ohm meter). Unless you want direction lights, you would make no connection to the topmost and bottommost two terminals.
Plus to one of the middle two of the six, ground to the other. Then the motor would be generally wired plus to left on one (say, the lower two), and plus to the right on the Upper one (for example). The other wire goes to the remaining terminal.
Usually that is done by connecting the bottom left to the top right terminal, and the bottom right to the top left. The motor is then connected to (for instance) the top two terminals. Remember, we are only talking about the middle six terminals.
With most DC motors, that will give one direction when up, and the other when down.
If that switch is "center off", which a turn signal switch would be expected to be, then there is a middle position which makes no connection. The description would then be "DPST center off". The "on-off-on" in the description means that switch should be a center off type.