You have an A/C truck, so there is at least one relay for high speed selection of the blower motor, and since it is a 72 there may be another relay for anti-dieseling (which is probably not needed. So, follow the wiring diagrams for the four seasons A/C which are in the service manual.
Service manuals:
http://67-72chevytrucks.com/vboard/s....php?p=5781194
Threads with A/C wiring page from service manual, both with and without anti-dieseling relays.
http://67-72chevytrucks.com/vboard/s...d.php?t=727010
https://67-72chevytrucks.com/vboard/...d.php?t=807883