I have to wonder at what point these repetitive ads diminish in value. How many times do you need to see a truck hauling a heavy load, slogging through mud to a husky voice over?
I don't think GM makes dealer commercials as they are the manufacturer.
Although they may not make the commercials directly, in print there is often "co-op" money available to dealers when certain criteria is meet by said ad... I imagine there is a similar system for television adverts.
Your descriptions sound like manufacturer commercials; Dealer commercials are the ones with jingles and usually the owner of the dealership yelling things at you for 30 seconds straight.