Power Sliding Door Opens but Wont close

Hello everyone, I have a 99 chevy venture and the Power sliding door open electrical but wont shut. I can here it click but wont move. I you try closing it maualy it come foward a little and jumps back to open position. Took it by the dealer for some warnety work. I had him give me estimate for fixing the door. He said it need a new wench to the tune of 960 dollars. He said the part was 699 dallors. I any one had this problem and fixed it, pleas let me know thanks