To the 2 or 3 of you 
who may have done this, where does the ball bearing go? I’m thinking on top of the spring, but don’t see why it would be needed there, so am not sure. Thanks!
Thon
On top of the spring for the detents.
How do you hold it together when reassembled?
David
+1 for spring position. When I did mine I drilled through and substituted the long rivet bits with long bolts worked well.
Con
Do you mean on top of the spring closest to the bottom of the picture? If so, still don’t really understand why. 
I don’t see how it could go on top of the detent spring (towards top of picture) as there is a plastic pin there, rather than a depression for a ball.
Sorry for the delay; I think it rides on the detents. Will look for pictures.
David
I’d suspect not, as there is no way–that I see-- for the ball to be moved from one detent to the next. There is a slightly domed plastic piece in the end of the spring, that I think does ride in the detents.
I didn’t make pictures two years ago and my drawing doesn’t show a ball. The plastic piece seems about right; I‘d put it together and find out what’s wrong with the functionality.
I have done, but couldn’t tell.
No one else been in one of these?
I found a magazine (XK-Files, music city jaguar club, August 2006). On page 12 they just say keeping the detent ball in place was tricky. The pictures provided are inconclusive.
It’s been too long. When I rebuilt the switch on my '83 XJ-S, the problem with the switch was there was too much stress on the contact panel and it got distorted. Wouldn’t park unless you used your fingers to lean the stalk switch a little in one direction. There were reports that brand new switches suffered the same problem. The fix involved fashioning a tiny aluminum beam to fit across the back of the contact panel to reinforce it. Used two long screws and nuts to hold the little beam in place, and I had to cut some of the webbing out of the housing to make room for the beam. Worked great, the switch clicked from one mode to another much more securely and made reliable contact in all modes.