I made this, it's better than a rheostat/baybus because those produce heat, cost more money, and can't handle big fans..
Read it here
Making a 7/12V Fan Switch Howto
Posted on Monday, May 26 2003 @ 16:12 CEST by Thomas De MaesschalckSpotTheCat made this howto for us here's a little snip
Read it here
Read it here