Most of the time it is the switch.
Try swapping the switches and see if that makes a difference. If it does, replace the bad switch.
If the electric motor continues to run after the window stops, you might take the interior panel off and check for an obstruction in the window mechanism.
It's not a huge deal - you just need to be careful not to scratch, bend or break any of the interior parts. The Haynes manual for the 123 is pretty good.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe