From our "Bah, humbug!" mailbag: In keeping with the yuletide spirit, MTV's latest contest--"Scrooge for a Day"--offers a lucky viewer the opportunity to be a network president. The tie-in with Paramount Pictures' release of "Scrooged," an updating of Charles Dickens' "A Christmas Carol" starring Bill Murray as an avaricious TV executive, allows the winner to become "rich, powerful and the greediest being on the planet." In addition to selecting MTV videos, editing the news and chairing programming meetings, the selected Scrooge will receive $25,000 in station advertising revenues and a percentage of the daily "Scrooged" box-office take not to exceed $10,000. The contest, to be announced today, runs until Nov. 21.