General Motors saw its credit rating lowered last week but financial problems at the giant automobile maker apparently won’t have an impact on its mortgage conduit and mortgage banking units. Standard & Poor’s joined other rating services in downgrading GM, and its GMAC finance unit, cutting the company’s long- and short-term credit ratings to junk status. The rating agency said the move reflects its conclusion that “management’s strategies may be ineffective in addressing [the company’s]…