Increase the profit margins. If GM is having cash flow problems, that means not enough profit. Raise the stickers to what they should be (about $3-5k more for every Pontiac model) and there you go. GM is always trying to be a "value leader" and the only way to be a profitable value leader is through volume. GM, and the NA market as a whole, will not get back to that volume business of yesteryear soon, if ever. Too much competition and choice exists for one or two models to sell 1,000,000 vehicles per year. If GM started making their vehicles with a slightly more premium mindset and increased margins accordingly, not only would they have a more aspirational (read: desired) product, but the profits would be up overall.
Truthfully, I'm at a loss of what exactly is "wrong" with Pontiac to the point it should be shut down. Pontiac has some great designs, and the only aspects of the vehicles that are lacking are the interiors. If the G6 received an interior update that is very similar to that of the new Malibu, I think sales would take off. You have a complete lineup, stylish design, and a gimmick: hard top convertible. The G6 should be selling a lot more than it is, and the only real detriment to its execution is the same interior (essentially) that plagued the 2003 Malibu. Look what happened to Malibu sales/demand for 2008. That wasn't due to the exterior update, as both were pretty inoffensive. The G6 at least already has a great exterior that's aged well. Give it a new interior and watch sales take off. Same thing happened to the SRX in 2007.