Why, when I turn a/c on, the air blows weakly from dash vents (even on high)?

Hello, you could have a concern with the temperature controls, the temperature blend doors, the mode doors in the dash, or a blower motor concern. This could also be related to a blower speed controller issue. Several possibilities, a trained technician with experience with Lincolns and full size Ford vehicles will be able to determine the cause of failure quite quickly. Sometimes it's helpful to take it to an experienced professional -- probably in this case. Here are some shops near you: https://repairpal.com/repair-shops/lincoln-repair-and-service-in-dallas-texas

It sounds like your blower motor has failed.