Hi,
I am in the exact boat as you are. I recently replaced the blower assembly with a second hand OEM blower in my 83' wagon to no avail and have checked the voltages across the blower motor/relay and resistor pack ect. I have concluded that it is just how they were back in the day. I wonder if an aftermarket blower motor such as this, would give a higher output of air;
http://www.ebay.com/itm/HVAC-Blower-Motor-4-Seasons-35398-fits-80-84-Subaru-DL-1-8L-H4-/151809519280?fits=Year%3A1983%7CMake%3ASubaru%7CModel%3ADL&hash=item23588d6eb0:g:BOoAAOSwT6pVrkvD&vxp=mtr
The hardest part is the blower motor is offset in the housing, so an adapter bracket will most likely be required to mount another blower motor. I have found that the early 2000 Subaru Foresters blower motors (w/o climate control) are fairly low profile and should work with a little modification.
Im interested in hearing what you come up with.
Regards,
Isaac