I am sure the dealer charged a packet for this option!
This is all you have to do, remove the plastic steering colunm surround, top and bottom, If you look at the the blinker switch, you will see a small white plastic clip, just remove it with a small screw driver, this is put there to stop the intermittent feature from working, all the relays wiring and switch is there, just the clip stops the wiper switch from going to intermittent.
here is some pics


I feel sory for the guy I bought my twin cab off, he could have had intermittent wipers all this time, for free!!