(FORTUNE Magazine) – Incoming Postmaster General Marvin Runyon earned his ''Carvin' Marvin'' nickname as chairman of the Tennessee Valley Authority, where he reduced the head count from 33,000 in 1988 to 20,000. He also won fans for holding down electricity costs. Repeating such successes at the U.S. Postal Service may prove tougher for Runyon, who prior to his four years at the TVA spent 37 at Ford Motor, rising from an hourly worker to a VP in charge of 29 plants and 120,000 workers, and seven with Nissan as CEO of U.S. manufacturing. Like the three PMGs who preceded him (see photos), Runyon will try to modernize and cut costs in the face of five strong unions with 650,000 members. Small wonder the job proved difficult to fill. When outgoing Anthony Frank asked around, one executive told him, ''Tony, I didn't know the depth of your hatred for me.'' Korn/Ferry, a search firm, finally lined up Runyon on behalf of the nine Postal governors, mostly business leaders. Why did the World War II Air Corps flight engineer sign on? ''I like challenge.'' Tips on the job include these from Frank: Make friends with Congress, which oversees pricing, and try to build bridges with unions, mail-order groups -- and Ralph Nader. Nader may resist. He has called Runyon ''a predictable appointment by a corporatist-dominated board of governors.''