After switching on the heating of the rear window, after a while the glass should be cleared of precipitation and ice.
If there are faults in the heating network, check the fuse first.
If the fuse is in order, check the reliability of the contact in the plug to the left and right of the glass. Clean contacts if necessary.
If the heated rear window still does not work, check the supply wires and the switch.
Check the health of the relay.
If the heating elements are damaged, the contacts can be restored using a special silver-based varnish (e.g. DODUCO, 7530 Pforzheim).