I was looking for a variable speed wiper solution so that I could retain the original switch. 

This looks like a reasonable solution using the original wiper switch. 
The have many other options for where you could use a variable switch 
http://www.revolutionelectronics.com/Products/

http://www.classicindustries.com/product/1967/impala/parts/re13009.html

75.00 plus+plus and exchange....

Anyone used this or something like it?