Easiest thing to do is get the repair manual for the van [available at auto parts stores or your local library] or ask a GM mechanic.
Heater fan resistors are inside the compartment where the fan squirrel cage (fan blade) is located to keep it cool. Look under the hood close to the fan motor for wires going into that compartment. Usually held with two small screws.
The resistor pack is mounted right next to the blower motor. You have to remove the insulator panel from the passenger side floor board area to get to the blower motor and the resistor pack. Once you have that off, the resistors and blower motor are near the firewall, and can be removed by taking out the screws that hold them in place. I believe there are 3 or 4 screws that hold the motor and 2 screws for the resistor pack.
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