Have a look on e bay, there are aircon heater combos small enough to fit under the dash and plumb in to the original vents.
About $180
I have 2 underdash units but would have gone for a combo if I hadnt already spent a fortune on original fit heater cores plus the unit I put in the wagon is big enough to freeze a bus.
Also there is no reason why one couldant fit an underdash unit elseware