There's aftermarket kits available that sells for about $200 US per pair of windows. It isn't difficult to install but time consuming. The basic install removes the window crank lever and attaches gears driven by a motor that cranks the windows up and down for you like the lever you removed. There are no modifications done to the car windows, sliders, etc... just the removal of the crank lever and install of the motor that will turn the spindle the crank lever attaches to. It does require the removal of the door panel as the install is done behind the panel so that it isn't unsightly and the kits do include covers to hide the hole left by the removal of the crank lever as well as all the switches and relays.
I've helped a co-worker install them onto the rear doors of a 325 sedan that had manual windows in the rear. Total time was about 4 hours for both doors as many smoke and beer breaks took place.