I had a look at the spare sliding door mechanism that I've been hoarding.
At the forward latch, there is a hook, spring loaded and slightly bent,
and a sliding "bar" directly above. When the door handle is turned, this
bar retracts a bit allowing the hook to move up in the female portion of
the lock on the body frame. The rear latch is opened at the same time via
the bowden (sp) tube/wire and the door pops out, driven by the spring on
that "u" hanger thingy. When the door is closed, the latching of the rear
catch causes the bar thing to slide forward on the hook, hopefully at the
same time the hook slides into the female bit. When the hook and bar are in
this position and out of the car, they can still move up and down, (they
are spring loaded down), but I guess that when in the female slot, there
is no room for both of them to move thus holding the hook down, but when the
bar retracts...
Gerry, maybe that bar is not moving in your case. You can activate the
whole lock shebang when the door is open by moving the rear latch
mechanism, see whats going on.
If you decide that you need to see more, warm up th e door panel before
attempting to remove it. I think its cold where you are now, and the
plastic bits that connect the "oh so fragile cardboard" door panel to the
door are a real bugger to pop out when cold.

Alistair