All,
DISCLAIMER !!! THIS IS NOT ADVICE, only what I did and the results. ATTEMPT AT YOUR OWN RISK. NO WARRANTY EXPRESSED OR IMPLIED. This is "internet info" and may/maynot have same results on your truck.

As previously stated I have a 13 Lariat 5.0 Screw and with the pax rear seat folded down about 5 o'clock & 3" from the seat latch on back of cab is a "lump" in the insulation. Behind the lump is a 2 wire connector. It has a 20 ga yellow wire that goes to a tab on the lower pax side of the rear window. It also has a 12 ga black wire going to a tab on the rear window adjacent to where the yellow wire connects.
I had previously lifted both yellow and black off the tabs on the rear window and still had mirror heat, but no rear window heat or indicator light on the control panel switch.
After looking at my wiring diagram it looks like the yellow lead eventually makes it's way to a box that says "defrost status". To an old guy, (me) that means on/off light on the switch.
Now I think I read on the "internet" that one or more SafeLite glass installers tie those two leads (yellow & black on my truck) together for some reason. I don't know, but I'm guessing that maybe an after market glass maker may only have one tab on the lower pax side of their window??? Anyway on my truck, both when I connect both tabs with a meter, "0" ohms show when def is off (meaning continunity between the tabs). When rear def is on 12+ volts on black lead, no voltage on yellow lead.
So figuring the worst I could do was blow up my 40amp fuse or make some sparks, I used a very small jumper wire and connected my black wire and yellow wire together, but NOT connected to rear window and turned on rear window def.
I got a light on the switch AND my mirrors started to get warm..
Turned rear def switch off, light went out and mirror cooled off. I only left "on" for 4-5 minutes, so don't yet know long term results.

Again this is not advice and only my experience with this issue. I'm also in N FL so I very rarely have frost on mirrors, but did want to see if I could provide heat to mirrors with an indicator light & not blow up rear window.
What I do not know is whether or not it will automatically shut off. (my 08 Silverado did after 10 minutes or so.)
UPDATE 12/30/16 18:15: Mine turned itself off in 15 minutes - outside air temp 50 degrees.

I will try this configuration for a few days and see if any bad things happen. If all goes well (as I think it will) may be a work around for the mirror heat & indicator light issue caused by disconnecting leads or pulling fuse used to heat rear window.
UPDATE 12/30/16 18:15 After connecting yellow & black wires, I insulated the connection & zip tied the wires to the window slider cable behind the rear cab insulation. Nice & neat and safely out of the way.

 

Very nice! I'm headed out to do mine now.

 

I did the "Shadetree43" mod and the insulation behind my rear seat caught on fire and my rear window exploded.

Just kidding, it worked just as described. Thanks!

 

Here's a few pics of Shadetree's rear slider window defroster bypass mod done on my 2014 XLT Screw Boost. With this mod, your truck's heated mirrors will heat and your in-dash rear defroster lamp will light, but your rear slider window defroster will not come on with the dash switch or with the remote start until you perchance to revert the mod somewhere down the road.

HEADS UP (edit) My method shown here is an overcomplicated way of doing this mod, but it demonstrates good wiring techniques that you could use elsewhere so I'm leaving it up. Shadetree has since documented a simpler method which doesn't involve modifying the harness. However, I think the cleanest overall way to do the mod would be to make a jumper of two small female spade connectors and a short piece of wire, then jump across the pins inside the OEM plug. Maybe the first person to attempt this technique will add a few pix to the topic...?

And here goes! I promise not to get as carried away with the documentation as I did with my DRL topic.

First up: You need to fold the passenger rear seatback down. This is a pic of the seatback release lever. The seatback is already folded down in this pic. Follow the red arrow to the little lever that has to be pulled upward to release the seatback. First you fold up the lower part of the seat, then you reach behind to unlock the back. Some folks use a box-end wrench to get leverage; I used my fingers. Next you release the lower part and finally fold the whole assembly down flat to the floor. It's really quite easy.




Here's a pic of the defroster connector, already disconnected for testing purposes. The slider crosses through the upper left quarter of the picture.




This is a pic of the lower half of the connector, removed from the back wall of the cab and prepped for Shadetree's mod in the way I like to do my wiring. I use a thermal wire stripper to peel back enough insulation to solder onto the bare wire. This technique won't damage the conductors in the slightest. You could also use a lighter to melt the insulation and a gentle touch with a pair of wire cutters to peel it away, but be careful or you'll nick the conductors .




Here's the 18ga jumper wire ready to be soldered. I used a little more wire than I needed so that I could simply cut and e-tape it off later if necessary, as opposed to digging back into the harness and desoldering old work.




And here's the finished work, with both lines individually e-taped and then taped together, and the connector remounted to the back cab wall. I simply wedged the other half of the connector behind the wire harness where it won't rattle loose. Done and done.

 

Or you can pull fuse 32 under the hood, which will disable just the rear defrost. Indicator light on the button will still work as will your heated mirrors.

 

Thanks for the post. What does the inside of the connection look like? Instead of stripping the wires and tieing into them, could you build a jumper that attaches to the inside of the connection?

 

My owners manual says Fuse 32 is "Rear window def relay power, Heated mirror relay". If I pull it, light in switch does not work. I wanted light in switch and mirror defrost to work.

 

All I did was take an old "add a circuit" fuse expander and plug the black wire & yellow wire into each other. Taped & zip tied to sliding window cable for security. Done - no cutting, no splicing, very easily restored to OEM.

 

It's fuse 32 on my 2010, not positive it's the same on the 11+. On mine it's called heated rear grid or something similar.

 

That didn't work on my '13 plat.