I think i have a fan problem

i think i have a fan problem, when i place 12 volts from the battery to the two different wires for the high and low speed and the speed barely changes. the fan humms slightly louder but not at all like our windstar which when the high speed kicks on in the van there is an extreme difference in speed
i was wondering if there were any more checks i could do to figure out if it is the fan motor that is shot and if it is what should i replace it with a stock replacement or a really good aftermarket one? is there a big difference in flow between an aftermarket fan vs. the stock one? and finaly could this cause my overheating because it only starts to overheat when it sits and idles up to like 40 or 45 ? because after i get up to speed like 55-60mph it runs fine temp. wise

thanks in advance

so just go to the local parts store for a fan or would it be good to get a high flow fan by someone like Flex-a-lite
i just want to make sure i get something good and reliable
thanks