I had difficulty finding out how to access the stalk switches. First release the steering wheel, lower it and pull it out telescopically. The wheel needs to be turned 90° left and right to get access to two screws. The top plastic cover can be prised off, starting by lifting the tabs covering the screws and levering upwards - see photo. (The top cover clips onto the bottom cover.) There is a third screw centrally underneath the lower cover. (6mm socket needed for all 3.) The separate cover over the ignition switch needs to be prised off and manoeuvred out. The stalk picture shows it half out with one locking-clip seen on top. The white connector on the PEC attaches the cable going up the stalk for the rear wiper control; remove it before unscrewing the PEC.
The stalk switches connect to the Body Control Module located behind the glove box; I was relieved to find that it was not the problem! The 3 positions of the wiper switch, Intermittent, normal & fast are encoded with resistors on the PEC, they are decoded by the BCM which commands the wiper relays.
Sadly I had already ordered a replacement stalk from Ebay before finding the corrosion. It may yet come in handy!