I've been told that the antenna adapter is no longer available at Walmart.com, however, one of my readers pointed out that it appears to be available from Crutchfield for $4.95.  The part number is 12040VW12, and here is the link:

http://www.crutchfield.com/S-pWNQxqzVlMq/cgi-bin/ProdView.asp?g=119400&id=morephotos&pi=2&i=12040VW12&display=XL