Your windows will work without the key so do mine ,normal with w124 .You do have a relay for windows .Regulators are a problem after a few years,and they do fail .Easy to fix .More of a problem is removimg the door car and refitting them back in place after.The metal gear wears out, but can be welded up to standard. Or just replace the whole thing .Just had a thought ,i have only replaced the fromt ones on my 124 .The backs are not installed on a metal frame with the motor , but on frame with cable type draw string and pullys ..