Hi Tom,
If it is a replacement cap (mines from B.D)  its a yakima key if you had your key the code would say A139 or what ever always seems to start A1??  If its a must open now and you can buy another one right now just insert big screw driver and turn with visegrips.  If you have a friend that is a lock smith or your good at drilling them out  if you get it unlocked and go to your friendly yakima dealer have them let you use a change key if its unlocked you can get the lock out and then see the core number  I can get you a key or you can order direct from yakima or replace the lock at $10.  If its an oem cap sorry I can't help. Just rember caps are cheap.
Good Luck
Kevin
81 Vanagon