02 Durango front and rear a/c do not work on the lower speeds -- only on high. a/c indicator light does not come on either, only on high. All pertinent fuses and relays are ok. How do I troubleshoot this further??

check the voltage at the blower motor. Unplug the blower, use a test light or voltmeter to check for power at the lower speeds (as well as the higher speeds). The light will be kinda dim at the lower speeds and increase in intensity (in the case of the voltmeter, the voltage will increase as the fan speed setting is increased).
If the blower gets voltage at all speeds, but does not work, suspect faulty blower motor. If it doesn't get voltage at all speeds, remove fan switch so that you can check each connection for power as the fan speed is moved from low to high.

Test the blower motor resistors (both the front and rear resistors).

The front blower motor resistor is located in the engine compartment on the right side of the firewall.

The rear blower motor resistor is located near the rear blower motor on the rear HVAC housing.

To test the resistors:

Disconnect the negative battery cable (place it so that it can't reconnect with the battery accidentally).
Disconnect the wire harness connector from the blower motor resistor.
Using an OHMMETER, Check for continuity between each of the blower motor switch input terminals of the resistro and the resistor output terminal. In each case there should be contnuity.
If OK, repair the wire harness circuits between the blower motor switch and the blower motor resistor or blower motor as required.
If not OK, replace the faulty blower motor resistor.

Good Luck.

It was the front resistor -- actually when I tested all pin combinations with an ohmmeter, there was a resistance between two of the pins. On the new one, all 5 pins showed connection between all. Original equipment resistor part is some kind of thick film solid state device. New part is all discrete. Thanks to all for replies.

Very good job Erv.