One thing I realized upon watching it again is that this episode is actually about three different stages in the radio business. The WKRP of Christmas past is an old-fashioned family-run radio station. The WKRP of Christmas present shows the family atmosphere being threatened by bottom-line considerations. And the WKRP of Christmas future is basically what radio stations are now and what they were already becoming in 1980 -- fully-automated and run by the sales department.
The music over the end credits is a vocal version of "Sleigh Ride"; there's another song playing in the Christmas past scene but I don't recognize it.
Cold Opening and Act 1