Tommy Boy's dead on the mark. You need your side window fan blowing in, and usually the ones in the back blow air out. If you don't have a front intake, you definitely should have the side window fan blowing in.
One of my friends, though, had one in the back blowing out (on top, since warm air rises) and one pulling in (the bottom one). It worked pretty well, if you can believe it. Note that he did not have a space to put one on the side window or he would've done that.