I’d buy my parts from a junk yard.
You’d still be supporting GM indirectly. In oversimplified terms:
If you buy parts from a junkyard, this tells the junkyard that these cars are desirable to purchase for the purpose of parting out, so the junkyard will buy more.
The junkyard buying more junked GM cars will cause increased scarcity of GM cars in the region and prices of GM cars will rise.
GM used car prices rising will enable GM to (eventually) crow about how their cars’ resale values are doing great in a TV ad (as several makes have done for various reasons).
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More directly, many junkyards track what parts get sold from what car. This information is sold to parts manufacturers and GM, who know what parts break often and can plan making more of them to sell in the aftermarket. This, of course, helps sales of parts to those that won’t go to junkyards...