New fan for back of case

Ok, so I just got a fan to use in the back of my case. My question is which way should it be blowing? I know that most setups have air sucked in through the front of the case and air blowing out the back, but my case has no front intake fan(not even a spot to put one). It's an Antec P182. So I have a fan under my heatsink which blows air through the heatsink to the top of the case and that air is blown out of the case by the fan on the top of the case. So would it be better to have that back fan suck cool air into the case and blow it toward the heatsink or should it suck hot air out of the case? Sorry if that's confusing and all, but if anyone could provide me with their opinions I would greatly appreciate it. If you don't understand all my jumbo about the airflow in my case you can google it and look at pictures if it helps.