For some unknown reason the base promaster does not have an armrest on the passenger seat. So I found a very similar one on Ebay and got busy installing it. I ended up making some brackets out of 1/8″ steel to place the mounting point in the same location as the driver seat armrest. I secured the brackets to the seat frame using some 1/4-20 bolts that I drilled and tapped into the frame. All in all it worked out good and looks just about identical to the factory one.
Here’s the Armrest that I used from Ebay.