The drivers middle air conditioning vent doesn't blow cold air - even the Pasenger's middle vent isn't that cold. Mechanics said it doesn't hold charge.

You do need a system full of refrigerant for your AC to blow cold air. If that's your issue, WHY it won't hold a charge is what you need to know. It's leaking from somewhere:compressor, condenser, evaporator, ac line, valve. . . your fix is to replace the leaking component, and whatever parts are customarily replaced along with it. Find a mechanic who will tell you WHY it won't hold a charge - ie, WHAT is leaking, what has to be replaced, and what kind of warranty will they provide? Good luck!