Funny you would write about this right now!!  I just had ours apart this
weekend because the two screws that secure it to the seat back were loose...
one was out completely.  It sounds like the locking pin on yours is missing.
Take a look and see if there is an empty hole that goes through the cylinder
the arm rotates on.  There is a 3/16's - 1/4 inch hollow pin that you have
to pound out to remove the arm.  Then I sprayed a little WD40 to loosen the
two cylinders enough to get one (connected to the arm) to slide off the
other (connected to the seat back).  Took me over an hour.  After I
tightened the screws and reinstalled the locking pin, the arm comes down now
but the mechanism apparently is bent out and the arm runs into the door
wall!!!  Oh well....

Madeline
15 Years of Westie Adventures
'90 Syncro Westie