[GMC Jim]

There are 2 relays and 1 resistor pack used with the Chev/GMC Four seasons air conditioning system....The blower motor relay is up underneath next to the outside top right hand side of glovebox. This relay has 5 wires total. Black ( ground); Purple (to blower motor); Orange ( to fuse panel); Dark Blue ( to resistor pack) and another Orange (18 guage) (to control switch)
The anti-dieseling relay is located on back side of instrument panel ; just to the left of the heater/air cond control. relay activates the air cond. compressor when the ignition is turned off ( to prevent after-run. This relay has 3 wires; Orange ( to blower motor relay); Pink ( to control switch) and Dark Green ( to compressor switch. Note:- my 1971, 402 engine does not have this anti-dieseling relay... possible former owner removed as it did cause some problems, I understand.
The resistor pack is located within the lower duct work.. To access; remove the screw attaching the floor outlet duct to the firewall. Thenremove the screws attaching the air distribtion box to the heater case. Slide this portion of the box downward and out of the way letting it rest on the floor panel. You should see the wires going to the plug to the resistor pack. There are 3 wires ; Yellow ( to control switch); Dark Blue ( to blower motor relay) and Light Blue ( to control switch)
This info is from the GM service manual........... Jim ( Hope it helps ! )