The New Yorker / Fifth Avenue names were shifted around a bit and it's easy to see why people are confused.
Fifth Avenue was a $1,647 package when the New Yorker name was transferred to the M body from the R body for 1982. In 1983 the series was called the New Yorker Fifth Avenue. In 1984 the New Yorker Fifth Avenue was split into two series with the New Yorker moving to the FWD E body. The RWD M body continued on as the Fifth Avenue for 1984. From this point the Fifth Avenue was considered the top of the line Chrysler, being both larger and more luxurious than the E body New Yorker.
For 1988 the E body New Yorker switched to the Dynasty body while the Fifth Avenue was available into the 1989 model year before being replaced by a new design based on the new Y body. The Y body was a 5" longer E body which was shared with another reincarnation of the Imperial for 1990.
Hope this helps.
Bill
Vancouver, BC