Just bought a 1991 Mustang and the a/c blows out only through the defroster,
does anyone know what the problem might be?

 

Could be a vacuum issue or missing equipment. Check out this diagram.
http://mustangstock.com/search/foxbo...um%20schematic

 

check the vaccum line from the firewall by the evaporator lines to source connection for a break

 

mjr46 has it .. the answer.

 

+1, I had the exact same problem

 

I checked the vaccum lines and i found a broken hose.Should I feel any kind of suction from the vacuum tree.

 

if the line is real small it'll be hard to feel it, hook a vaccum gauge to it to read actual in.hg

 

while letting off the clutch and easing into the gas the gears slam.Could my rear gears be worn or is it something in the transmission.Also when I am in 4th and 5th gear i hear a humming.I dont hear it if i rev up the engine in neutral.

 

could have bad bearings in the tranny and for the slamming, check the u-joints