I don't think that using a variable resistor to vary the wiper motor speed is a good idea.
It will probably cause the motor to overheat.
If that's a 500K Ohm resistor, it would be way too much resistance even if it would work.
Plus, the resistor would have be rated at about 200 watts (16 Amps @ 12V), and that is a pretty big resistor.

If I was going to do this (and I'm not), I would use a timed contact to turn the wiper motor off and on at a selected interval.
I would by-pass the existing delay relay and install the contact in the wire from the wiper switch to the motor.
The power wire from the wiper switch is white, and the wire to the wiper motor is white/black.
I would place the timer contact between these two wires.
That's what I would do if I was going to try it.
Of course, I wouldn't take a chance on my wipers working dependably.
It wouldn't be good to be on the freeway in a driving rain and suddenly lose your wipers.

steve