DETROIT — Chrysler is eagerly awaiting the arrival in 2012 and 2013 of promising new vehicles based on technology from its new partner, Fiat. For now, though, the automaker is still struggling badly. Chrysler's sales fell more than any other major car company last month, as they have for most of the year — down 26 percent last month while the overall United States market was flat. Its market share has dwindled from nearly 11 percent in the months before its May 1 bankruptcy filing to about 8 percent after it. So Chrysler is trying to market its way back. It fired its ad agency, drastically altered the brand messages for Jeep sport utility vehicles and Ram pickups, and it is offering big incentives to clear dealer lots of inventory. But after receiving nearly $13 billion in federal loans and cleaning up its balance sheet in Chapter 11, it is struggling with products that trail the competition in quality, styling and fuel economy.