This is for the heater core.... http://www.jeepforum.com/forum/f12/heater-core-219329/
once you're that far the fan motor is self explanitory.
I would HIGHLY recommend you do that while you're in there.
I'll just put this here before someone comes in and tells you what an awesome modification a chevy blazer blower motor is..... Seimens PM102 is the replacement stock YJ part number for the fan, oddly enough it's the exact same number as the blazer motor "upgrade". At one time this was an upgrade for early model YJ's (pre '92) not so much now, they're all the same motor.