Hey GM, if you're gonna shrink, shrink your ad budget.

Get rid of your logos. Your taglines. Your campaigns. Your brand equity. Become the car company of the future. Keep Cadillac, maybe Saturn separate... but no more buick, chevy, geo, pontiac, etc.
Now it's all GM. One hood ornament. One logo. One ad look. One agency. Slim down the messaging and the red tape of delivering said messaging. Corvettes are still Corvettes if they're GM. It's just a fuel/energy/financially efficient way to keep your company afloat while honoring the history... and embracing a future. There can be one website. One boss. One marketing team. The subsequent savings would be visible.