Excellent post, a great solution.
However, there is an accessible solution for all us ham handed with a soldering iron.
There is a relay that does this when plugged into a standard wiper socket. Click intermittent on/off once, then wait a while and turn on intermittent. This measures and copies the interval (I think 1-30 seconds)
The one I have is DUDOCO - Interval Chip - 3579 0010 01
Here is a similar product - http://www.autopartswarehouse.com/intermittent_wiper_relay~pop.html
Mine was bought a long time ago for an Audi and works fine in my defender.