These used to be common aftermarket add on that you could buy
from anywhere from K Mart to Repco.  Similarly electronic
magazines like Electronic Australia (no longer published),
Silicon Chip, Elector etc used publish articles on how to make
them and Altronics, Jaycar and Dick Smith used to sell kits so
you could build your own.  There was one in the Sept 1979,
November 75 editions of Electronics Australia and May 1990
Silicon Chip for example.  But as they became standard with new
cars they have vanished from the market and electronic magazines
and stores.

They are not hard to build even for a novice so I suggest you
get hold of a circuit from http://www.siliconchip.com.au/  in
their indexes (they  also have the old EA circuits
http://www.electronicsaustralia.com.au/   and
http://www.electronicsaustralia.com.au/cgi-bin/downloads.pl?area=4)
or  see
http://www.electronicsaustralia.com.au/cgi-bin/downloads.pl?area=4 .

For one on line see http://netti.nic.fi/~graytech/AdapWipe.htm
or http://www.rowand.net/Shop/Tech/DelayWipers.htm for example
or just search Google for pulse or variable speed wiper
circuits.  The hardware to build one is still readily available
from Altronics and Jaycar.