Part of It should be located under the battery tray - the other half of it (the fan part) is inside the dash.
You have to take part of the dash on the passenger side out to get to the heater core - which you might as well replace (the heater core) while your there! And if you want, you can research about upgrading your blower motor while your at it. I did this 2 winters ago - great mod!
I did them both at the same time on my '87. Not too hard, just time consuming.