Water accumulating in the rear passenger floor area. No sun roof.

Hello, The membrane that separates the door panel from the door may be leaking, or, like your mechanic checked, the floor may be leaking. There is nothing else that should be able to put water into the rear floorboard, but the HVAC clean-out should be checked for clogging higher up the tube. If you have additional questions, let me know, and here is a list of certified shops in your area, for your convenience: http://repairpal.com/lincoln-within-10-miles-of-point-pleasant-west-virginia-25550

I had the same problem, there is a seal that goes bad underneath the passenger side windshield wiper, not hard to get to from the outside, I replaced the seal on mine with some replacement sealant material and it stopped my leak.