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As it is getting more colder and car is covered with moisture in the morning I noticed that the rear window heater seems to be a little weak... It heats up, all wires are working but still after 15 min drive only small clear lines have appeared in the window. Both mirrors are already pretty hot by that time . As this 1995 XJR (and Jaguar in generally) is new car to me I cant compare is this normal or not. Maybe X300 rear glass heater is a bit weak and its normal? Maybe some long time owner can comment on that? And if it is not normal behavior, then where to start looking for problem?
Since the system is working (though not well), all I can think of to check is the wiring connectors in the circuit and the fuse and relay for good contact.
Fuse F15 in the trunk is a 25A fuse so there could be 15-20 amps going through the relay. The relay is the leftmost one in the trunk, with the yellow base. Perhaps switch it with the red base relay (brake lights) if unsure about its ability to carry the current. Check the connectors at the rear window too.
It could be that the rear window heater is normally a little weak. I can't really vouch for it since I hardly ever use it in Arizona!
I think the advice to check and clean all connectors is good; there's going to be a poor contact and consequent voltage drop somewhere that's causing the heater to be weak.
The X300 rear screen heaters are completely "normal" in normal operation, and are comparable with pretty much every other similarly equipped car. Check the connectors as advised, as there isnt too much else to worry about. There is a timer which switches the window off after a while, but it usually allows plenty of time to clear the window.
Well... Now I screwed big time. I was checking trunk fuse box for connections like you suggested. I pulled out fuse, which was fine, cleaned contacts and put it back. Then I wanted to test the relay and took out the multimeter. Relay was working fine but during the testing I managed in some idiotic way connect the relay power input and one of the relay control contacts with multimeter. It was just a bad movement of my hand and it touched wrong contact in fraction of second. The relay made strange noise. I went inside the car and smelled something burning. I immediately switched off power. I did not understood where the smell come so after some checking I turned on the power. No burnt fuses in trunk or RH rear foot fuse box. All relays are working. Car does not have any faults in display and every electrical device is working as it should. Except rear window heater. If I switch it on then the mirrors start to heat up but nothing happens with rear window heater now.
Before the screwup I could see that when I press the Rear heater button, the dashboard voltmeter needle will drop a little and then rise back. Now there is no change in voltmeter which indicates that no power is sent to window. The relay is working (I can switch it to different position and it works) but it seems that there is not coming any input signal from the front button to turn the relay on?! I think I accidently feed power to some control module and destroyed it so now the push of the button is not sending signal out... The button lights up, mirrors start to heat but no signal sent to rear relay.
Weak rear window heater
. As this 1995 XJR (and Jaguar in generally) is new car to me I cant compare is this normal or not. Maybe X300 rear glass heater is a bit weak and its normal? Maybe some long time owner can comment on that? And if it is not normal behavior, then where to start looking for problem? 
