General Motors Co (GM.N) has turned to two investment banks to help it come up with a response to a shareholder group demanding $8 billion in stock buybacks and a seat on the board of the No. 1 U.S. automaker, the company told Reuters. The automaker is working with Morgan Stanley (MS.N) and Goldman Sachs Group Inc (GS.N) for advice on how it should deal with former U.S. auto task force member Harry Wilson and four hedge funds that are pushing for the company to dip into its $25 billion in cash, GM said on Thursday.