Looking at the 72 service manual wiring diagrams, which are a little hard to read but available here:
https://67-72chevytrucks.com/vboard/...d.php?t=558016
It looks to me like in the cab that position on the connector next to the purple wire is a brown wire that goes to the radio fuse. So, with key on, remove the radio fuse (carefully) to verify the brown/white wire in the engine compartment that comes out of the connector next to the purple wire loses power.
I think it would work fine for an electric choke. If it blows the 3A fuse, insert a 5A fuse. I don't think it blow at 3A will because I have powered my electric choke off the radio position and it worked fine, but I didn't have the AM radio blasting either.
Too-long-didn't-read info below. Really it's more than you want to know. My advice is don't read it. Stop reading. Are you still reading? You are getting really sleepy...
Instructions for reading the wiring diagram, from the manual (all upper case courtesy of Chevy):
THIS DOCUMENT INCORPORATES A NEW MAPPING CONCEPT TO REPLACE LINES REPRESENTING WIRES. IT READS IDENTICAL TO MOST ROAD MAPS IN THAT NUMERIC STATIONS ARE ASSIGNED
HORIZONTALLY AND ALPHABETIC STATIONS ARE ASSIGNED VERTICALLY. THEREFORE ANY COMBINATION OF NUMERIC AND ALPHABETIC CHARACTERS DESIGNATES A PRECISE LOCATION.
TO ILLUSTRATE THE USE OF THIS CONCEPT LET'S USE THE HORN CIRCUIT. LOCATED IN THE UPPER LEFT HAND CORNER. AS AN EXAMPLE. THE NUMBER 29 CONTAINED IN THE RECTANGLE [29] REPRESENTS THE CIRCUIT NUMBER. IMMEDIATELY TO THE RIGHT APPEARS 12DG. THE NUMBER 12 SPECIFIES THE GAUGE OF THE WIRE AND DG IS THE ABBREVIATION FOR THE WIRE COLOR. DARK GREEN. SHOWN IN PARENTHESIS IS (24C). THIS IS THE ADDRESS IDENTIFYING THE LOCATION OF THE OTHER END OF THE WIRE. TO FIND THIS LOCATION. SIMPLY SCAN THE BOTTOM OF THE SHEET TO THE NUMBER 24 AND THEN SCAN VERTICALLY TO THE APPROXIMATE LOCATION OF C. THE CIRCUIT 29 SHOULD POP INTO VIEW.*
TO FOLLOW THE CIRCUIT IN REVERSE, SIMPLY REVERSE THE PROCEDURE. FOLLOW THE LEAD IN WIRE OUT OF THE RECTANGLE [23] AT THE END OF THIS WIRE APPEARS (4S) 12DG AGAIN, THE NUMBER 12 REPRESENTS WIRE GAUGE AND DG THE WIRE COLOR. DARK GREEN. (4S) IS THE ADDRESS. TO FIND THIS LOCATION. SCAN THE BOTTOM OF THE SHEET TO STATION 4 AND SCAN VERTICALLY TO THE APPROXIMATE LOCATION OF S THE CIRCUIT 29 SHOULD BE IMMEDIATELY APPARENT.
* it's like magic! Try it, you'll just love it.
part of engine compartment
part of in-cab wiring
To view in better resolution download the 72 service manual or have a look at a paper copy which are available for cheap from any of the catalog vendors or ebay/amazon.
Fuse panel info:
https://67-72chevytrucks.com/vboard/...d.php?t=741245