According to the ZDF web-site, Frida apparently decided to lend her voice to "The Christmas Experiment" spontaneously which was originally an instrumental piece.
Frida joined all the other performers on the show at the end and joined
them for a German Christmas Song.
"The Sun Will Shine Again"
"In The Bleak Mid Winter"
Thanks to Pieter Wiever, Robert Verbeek, Lex Corbach and Erik Menkens c/o ABBAMAIL and Mats Butenschön. Thanks also to ABBA Remember, Ronald and Philippe Dupont.