Did a search here for the most likely culprit, found nothing so I have to ask.
If the rear defroster stops working, but the button works (it lights up when I press it, just nothing happens on the rear window), that would mean the relay is OK? That leaves only fuse or actual wiring as the possible problem, I imagine?
Where is the fuse for the rear defroster? And where does the wiring go from the button to the rear - through left or right side? Through the floor or doors or roof or elsewhere?
Any help appreciated.
Also, does the N. American 1990 LS400 have a defroster built into the side mirrors as well?

 

So I inspected the rear window heating wires and there is absolutely no break in them! I also don't see the connector anywhere - it must be hidden somewhere under the roof lining or something... but that means that no one could touch it to disconnect it accidentally.
Considering that relay (probably) works since the button for rear defogger lights up, and that there are no breaks in the heater wires, and no visible loss of contact, what am I to look for next? Fuse? Wires that run along the car? where are they?
Any help appreciated. (Yamae, are you around?)

 

Need to check for power right at the defogger connectors one on each side of the glass. Sometimes the terminal falls off the glass if so this is repairable by using silver laced epoxy.
https://www.mgchemicals.com/products.../silver-epoxy/

There are 3 circuit breakers left hand fuse box under the dash they are reset by pressing them with a pin one could be tripped. They have a silver top with a hole in the middle or on the edge. Some have a green top I think.

....oh and speaking of green the defog relay is green, if that is for the rear defogger lines not 100% sure.

 

I would check the relay and the 50A fuse for the defroster first. As far as I know, the indicator bulb is driven together with the relay coil not together with the heating element. I don't have a manual for a 90 and I'm writing this seeing the diagrams for a UCF20 1995 and a 2000. So there may be some differences but a 95 and a 00 are almost the same and I imagine a 90 should be similar. In case of UCF20, both the relay and the fuse are located in the big fuse box behind the battery and I'm sure you can find and check them using a multimeter easily. Attached below is the relay's pin assignments.

 

Thank you Yamae and LrX2K. One thing came to mind: lately my trunk has been left full to the brim... (hiding gifts and such). Could it be that one of the packages knocked out the rear heater wire (if that wire goes through the trunk in the first place)?

 

I had this issue two seasons ago. My problem was with the connector no longer making a good connection where it attaches to the defroster coil. The connection is hidden under the plastic housing on the drivers side of the rear window. I sourced new clips on line but could have gotten by with the original.