no heat blowing from vents

Is this the fault of the mechanic when he installed the motor? how does this get fixed/how much usually?? Thank you

cheap fix.look under the hood,passenger side near firewall first.usually the smaller hard plastic lines,crack,split,dry rot.start the engine turn the heat or air on vents,set to high,listen for hissing noise,repair the leaking vacuum line.unless mechanic disconnected a line under the dash,then most likely under the hood.vacuum hose and fittings to fix it/them are cheap.

What do you mean by firewall?