My rear back window won't defrost

Are you sure that is not the AC compressor relay?
I don't know your car, but not aware of any cars having a defroster relay, a defogger relay for the rear window with the electric grid in the window, yes. Never heard of a defroster relay.

There are 2 Plugs that go into the REAR Defrost. If one of them is off, then it will not work. If you broke the Electrical Track, then it needs to be repaired... They sell kits to do this at your local Automotive Repair Stores... Time consuming but easy to do.

Hi Sabe, I'm glad to help!

First go to the rear window glass and check power and ground at the glass connectors, also make sure they are connected to the grid because if they are loose then you have found the problem. If they are tight and you have no power to the connector then check fuses. If the fuses are good you'll need to find out why your loosing power between the fuse and the rear glass connector. It could be another connector somewhere in between. Hope this helps and have an awesome day Sabe.

Look at your maintenance manual. To find where your fuse box is located(may be under the hood or under the steering wheel). When you find the fuse box look on the back of the panel flap and you'll find a fuse map. Locate the fuse that corresponds with your problem. It could be the same fuse but not likely. If fuse is blown then replace it with a spare. Most manufacturers will put spare fuses in the box or you could pull a fuse from some where else just to try and see if the problem is still there( make sure it's the same size and understand that it could blow this fuse as well then you will need 2 fuses). Most of the time fuses don't just blow. For some reason it pulled to much load. Probably a dead short in the wiring. I hope this helps.

Duplicate question

Hi Kolya, Check around for a loose or broken cable especially around the battery connections. Regards John

Yeah, I'd think it would have lines, too, unless some new way of doing it that I'm not aware of. Maybe the rear window was replaced with a window without the defrost option. There should be a small two wire connector coming out of the back shelf near lower driver's side of rear window. That would be the defrost connector to the window lines (that aren't there,lol).
Well, good luck with that! I think there are aftermarket kits for rear window defrost, though.

You definatly have water inside the car. Pull the carpet back in the front and rear checking for water leaks. The only way frost can develope inside a vehicle is the presents of moisture.
The other possibility is your HVAC drain is plugged and water has built up inside the heater motor area. Have the drain checked and cleaned.

yOUR HEATER CORE NEEDS TO BE REPLACED. ANTIFREEZE CONTINUALLY RUNS THROUGH YOUR HEATER CORE WITH OR WITHOUT THE HEAT OR AC BEING ON. THIS ALLOWS YOUR ANTI-FREEZE TO COOL ABIT WHEN IT RETURNS TO THE BLOCK.

During the restoration of our '92 Lesabre, we had an auto glass guy come out to remove & hopefully replace the same glass in the rear window section(the window trim had disintegrated and we bought new stuff to replace it with, but it wouldn't fit unless the window was removed).
What was discovered when the window was removed... was that the defroster interface on the sides of the glass was basically disconnected from the glass: the solder contact patch where the power & ground wires interfaced with the defrosting wires was corroded. It fell off once it was pulled on. In our case we had the whole rear window replaced.

What may be possible for you to do is pull back/off the rear pillar trim that covers the defrost wire & connectors and re-melt the solder with a 80-150 watt soldering iron(assuming that is where the problem is). I suppose, before buying and/or firing up the soldering iron, get the rear trim that covers the left & right edges of the rear-glass removed to find out IF the connectors & the long vertical strip they attach to are indeed attached to or detached from the glass. Oh duh, the vertical strip that the power wires connect to on either side run from top to bottom of the defroster array and are the portions that solder to the glass.

If it results in soldering, I'd suggest you use either a resin/flux core electrical solder or use a flux paste brushed over the solder contact areas & use solid electrical solder.

If I wasn't so busy with other parts of the car I would have gone after the soldering myself... but being as pre-occupied as I was that's why we opted to have the whole window(which includes the defrosting array & connectors) replaced.
I hope things work out for you regarding this... shoot me back if you need any other info/follow-up.