It'* possible that the passenger'* airbag makes access to the airmix actuator a bit harder than on my car, that only has the driver'* airbag.
However, you may want to try to remove that lower plastic panel on the passenger'* side, just above the passenger'* feet, if you haven't already. Then you might be able to reach the actuator from below with your hand.
Take a look at this photo, I took it while working on my car. The heater core is that radiator on the right, normally there'* a black plastic case that covers it, with the airmix motor mounted right on top of it. That metal rod just above the heater core, controls the airmix door. The motor rotates an arm that connects to that rod. You might be able to reach it from below.
My car is a '92, so yours may be a little bit different, but I think the general design is the same. Here are the links (you may need to open a new browser window and copy&paste them):
Hope this helps, let us know how it goes.