Dave,
On the 126 420SEL, continue from the passenger compartment. There is a screw under the door side panel in the footwell, which must be removed first in order to remove the knee panel. After the knee kick panel is out ther are three? screws to remove on the blower case bottom and a couple of clips towards the front of the bottom. Easier than it sounds, but the clips are a little tricky the first time.
Once the bottom is off, you're looking at the blower. Removal is intruitive from there. Either lube the bushings with AT fluid or replace the motor.
You'll not get there from the firewall on your car.
The 190 series can be accessed from the firewall to the heater core area, but not the 126.