Cars cost only $20,000 on the average. If somebody buys a $30,000 SUV and default on it two years later. You can recover at least $20,000 unless gas prices shoot to the moon. It is easier to sell a car than a home. The downside risks are much lower.
You need to weigh the tradeoff between auto sales and credit line to GMAC. I would rather spend $$$ on GMAC rather than bailing out GM/Chrysler/Ford. Ideally I would prefer neither.