Installation is easy: Simply remove the drivers side kick panel and locate the factory Volvo wiper relay(#203). Remove the #203 relay and install the 99 relay in its place.
To use the adjustable feature, set your wipers to intermittent and turn them back to off before a full sweep has been completed. After a desired time, set them back to intermittent and leave the stalk there. The time between these two settings is saved and repeated until you move the stalk to another position. Once car car is turned off, it reverts back to the 6-second interval.
I have more than one of these for sale and you may receive a relay that has a different part number than pictured. It is becauses VW changed the part number at least twice over the years. Functionality is the same across all part numbers as long as it says 99 on top. It is a factory part and not aftermarket.
Here's is a youtube video describing the wiper relay's operation:
http://www.youtube.com/watch?v=zCHp2Qg_ngI&t=6m47s
For 850 and 70 Series up to 2000
$15 with free shipping and delivery confirmation to the lower 48.