On my'92 (which should be like yours), I had to pull the heater box out. The fan motor was attached to the heater box and the motor sticks through the firewall.
This is not a fun job...
Do yourself a favor and put in a new heater core while you've got it apart.
“The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed.” ― Robert M. Pirsig, Zen And The Art Of Motorcycle Maintenance: An Inquiry Into Values