1986.5 Toyota Supra heater fan replaced but not a lot of air coming out of the vents

I replaced the heater blower on my brother's 1986.5 Toyota Supra after his blower no longer came on. The blower is on high but there isn't a lot of air flow coming out of all the vents. There are three settings low, med, and hi but low and auto will not turn the blower on. It will turn on if med and hi are used. The blower motor resister needs to be replaced if this is the case.

If I replace the resister will that allow more air to come out of the vents and allow the blower to increase in speed or is there another problem like clogged air vents or dirty air filter that may be the culprit?

There is air and heat being generated out of all the vents just not a large amount. It feels as if the high setting is only generated a low to medium amount of air. I own a 1988 Toyota Supra and when I turn the blower on high there is a ton of air at a high speed coming out of all the vents.