I am assuming that air is coming out of when your car is not moving (means fan is working). The reason, I am putting so much emphasis on it, is that you can have bad fan but when you drive the car fresh air can come through and you will have hot or cold air but not enough. Here are couple of things to check.
1- When AC on and small amount of air coming from the vent, check for the defrost vents. Do you have a lot of air coming out of the defrost vents? If that is the case it is your vacuum element.
2- When fan set on high speed, do you hear the fan noise (usually loud). If not most probably you fan brushes are worn out.
3- Last but not least, your ACCU is bad. Might have bad solder joints.

4- Try applying direct 12V to the Fan motor to see if it runs on good speed and does air volume is good through the vents.

I hope this helps, keep us posted.
Thanks

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87 420 SEL
Looking for a nice mid 80's MB diesel.
87 BMW 528e (Sold)
99 TOYOTA 4Runner
1989 Honda Accord (Sold)
1988 Honda Accord (Total)