The heated rear window is switched on by a switch located on the instrument panel. The current consumed by the heater is large enough, so after defrosting the rear window, turn off the heater.
The grid of conductive filaments of the glass heater, applied by screen printing on the inside of the glass, can be damaged by careless handling, as a result of which the corresponding part of the electrical circuit may fail.
The exact location of the break can be determined with a voltmeter.
These faults can be repaired using Rear Defogger Repair Varnish, catalog number MPR 77 01 421 135 (packaging 2 g).