My '01 EV MV had the same issue, but with the driver's armrests.
The plastic cap pops off with a little effort and without tools. Underneath is a 6mm hex head bolt that you can snug up.
Since it's recessed a bit, and since I don't own T-handled hex wrenches, I used the long side of my wrench and put an adjustable wrench on the end.
If you see a silver EV MV equipped with Yakima racks in Mission Viejo tomorrow, wave to me.