RCN Corp. looks like it may scoop up Ameritech's Midwest cable overbuild. Industry executives said the RCN is in the final stages of negotiations, haggling over pricing of the systems. Ameritech parent SBC tapped investment banker Morgan Stanley Dean Witter last month. It's not clear whether Morgan has gotten RCN to raise its bid, or SBC has substantially lowered its asking price. RCN, which is overbuilding both cable operators and telcos in Chicago, New York, Washington and Boston, will have to stretch hard to complete the Ameritech systems and refit them for telephone traffic, which will, of course, mean that Ameritech will be creating a new competitor.