If you apply voltage to the switch contacts without removing battery positive post, you may fry the voltage regulator.
DO NOT LEAVE THE KEY ON FOR MORE THAN A BRIEF PERIOD OF TIME WHILE TESTING WINDOWS IF BATTERY IS CONNECTED OR YOU WILL FRY THE VOLTAGE REGULATOR. Ya, I know ... I had a spare.
If you know the windows did work at one time and no one has modified them since that time (I have seen Ebay cars with wood supporting windows!) and jumping the switch does not work, remove the door panel and inspect the regulator.
Warning: If the motor works and the regulator slider is faulty, you can on some models force the slider rivet or pin into the aluminum rails, damaging the slider rails by operating the switch / regulator motor and you will have to remove the rails to file smooth the damage or you will be replacing sliders often.
If you have to dissemble the front doors to remove / replace the regulator, study shop drawings and instructions first. You have to remove the arm rest, door lock lever plastic 'shroud' & metal 'housing', plastic lock pull knob, chrome lock plate, vertical door trim, inner panel (remembering the light on the bottom of the door), drop the window (heavy ag. silo tape helps hold window and will not leave residue like duct tape, you will have to 'hold' the window in several positions) remove the rubber door/window seal, mirror, two small screws holding the vertical window rail, remove the rail with a twisting motion (some prefer to remove the outer trim first) ... get the point? You can do this but you can also reassemble the door in the wrong order and start over. DO not remove the seat switch cover or you will have small parts everywhere!!!!!
While in there, lubricate the window with MB lube or Lubriplate (get a glob from an engine rebuilder) and remove any wheel bearing grease (!@&#%@% PO fool!), clean and lubriplate the switches, verify vacuum lock gizmo with a mini vac, verify door water weep holes, look for any parts inside the door to give you a hint on pervious work (?) done on the door, replace the lock vacuum rubber connectors, adjust the regulator stop, graphite lube the lock ... Keep a mechanic's magnet handy and a clean ice cream pail for the small screws and after removing the inner door panel from both doors, work on one door at a time so you have a reference to go by.
I know this sounds like a very large job, but if you do everything on the door you will not have to get in there again for a LONG time. I have replaced sliders only to have the vacuum line connector fail, then ... get the point?
If you are not comfortable doing this, practice on a bone yard car but please replace all hardware!
Don't you just love how I said 'remove the mirror' not mentioning all the bolts, screws, seals, wire connection ... maybe I should have said 'disassemble the door and repair any noticeable damage'.
When done, turn toward Germany and shout "why didn't you make millions of 240Ds with the 617 turbo engine, manual windows, manual trany, manual locks ... a MB folkswagon".