LONDON/MILAN/DETROIT (Reuters) – General Motors (GM.N) and Fiat Chrysler Automobiles (FCA) (FCHA.MI) have turned to investment banks for help to deal with a stand-off as FCA seeks to force a merger with its Detroit-based rival, according to sources familiar with the matter. GM is being advised by Goldman Sachs (GS.N), while FCA is working with UBS (UBSG.VX) on the matter, several sources said, with one adding that Morgan Stanley (MS.N) was also working with GM.