I just completed wiring up my 2015 for retained window power where I can stop, turn the car off and have time to put up or down the windows before the relay cuts power to them. In the broader car world this called Retained Accessory Power (RAP.) I used one Beuler time-delay relay and one diode. I cut the green wire between the ign switch and the power window relay. The following is a list of connections:
86 (coil voltage in) -- to cut green wire going back to ign switch -- diode is inline here between coil in and switch wire
85 (coil out) -- ground
87 (relay contact out) -- to cut green wire going to power window relay
30 (relay contact in) -- always hot contact inside MB (fuse box under hood)
With the Beuler time-delay relay I can set the time for how long I want to maintain power. I set mine at 35-40 seconds. It works. I have seen other posts about this but none that say they actually implemented it and got it to work. Mine is done and working. I will add a diagram later when I get a chance in case anyone else wants to do this. But be warned it took a while and you have to be persistent to get it done right. I did it over a couple weeks working a little bit each night and a couple Saturdays a little. Actually hooking up the wires is not hard; it is finding the right wires in the car, finding a ground point, actually running a whole new hot wire from under the hood to near the cabin fuse box in my case (quite a chore and this is where I had most of the front fender off but found I only needed to remove the plastic wheel well liner later,) that takes time. So while I will post a diagram, I will not go into the specifics of where I connected everything on the car unless someone really honestly plans to do this. And if you do, you may have to message me as I may not look back at this post; getting forgetful.
Life is good now..........