DETROIT - In a bid to reverse its fast-declining share of the North American market, General Motors has instigated a top-level marketing shakeup.
    Out goes vp-marketing Brent Dewar (50), in comes the former general manager of GM's western region, Mike Jackson (49), his appointment effective March 1.
    Dewar becomes VP-field sales, service and parts, succeeding Pete Gerosa (64) who assumes special assignment duties until his retirement later this year.