Bill, is correct about the "pusher" & "puller" info however, depending on the fan that you are using you may cause damage to the fan motor or electronics by reversing the wiring. It's best to check with the manufacturer for info on changing a pusher to a puller. Here is a link to Flex-a-lite's fans webs site FAQ section. It provides some good basic info on fans.
http://www.flex-a-lite.com/auto/faq.htm
BTW: I have Flex-a-lite's "Blackmagic" and it works great. Other sources for fan mfg. links can be found on Summitracing and Jeg's web sites.
good luck....keep cool,
mike "monster"