A relay works well for the single speed and auto park function. Here is the diagram with an on/off/on toggle which will have hi & lo speeds.
to use a on/off switch with one speed use a simple 12v relay as follows (on the relay):
30 - main power supply
85 - ground
86 - power from switch
87a - to 53A on wiper motor
87 - to 53b on wiper motor