I've done it on my old 745T. I used 4 850/960 autodown relays, one for each window and added a reversing switch to auto-up the drivers window which comes in handy at tollbooths or if you need to spit your gum out in the winter. It's not hard to do IF you have an OEM wiring diagram (800-25-volvo) and some understanding of basic auto-electronics. To add the reversing switch for auto-up, I used an A/B toggle switch from radio shack which reverses the current the moment I flip the switch. I mounted the reversing switch to the door panel just above the other window switches. One could easily do the same door mounted reversing switch to an 850 as well with the door panel off and the wiring diagram handy.