You will have to work from the inside out to get to the o-ring and the spring washer. You have to remove the crank then you will see a nut around the shaft about 3 or 4in maybe more up in the roof. I made a tool like this
http://www.swidenonline.com/s-271-winegard-antenna-parts.aspx
out of 1/2" PVC white pipe screwed a 1/2" bolt in one end and used a bench grinder on the other end to fit a socket but I can't remember the size of the socket, something like a 1" Don't get the nut to tight when going back together or you will not be able to turn it left or right when up.
Hope this helps
JR