I think that you are going to need to come up with some kind of timer module to insert in the circuit of the intermittent section of your wipers. You might want to look at one of the Ford timer modules and see if you could wire it in to your circuitry. The Ford unit is pretty simple, and they are plentiful at the salvage yards. You do not want to buy one new (don't ask me how I know this).

If you are good with electronics, and can figure which wire sends the signal to the wipers, you might be able to wire a pot in so that you could hurry or delay the signal to the motor.

Bill