I did a blower motor replacement on a 1987 300D, very similar to the car you are talking about as far as the placement of the major items.

The motor is likely squealing because the bearings are worn, not to mention those brushes. So, a motor replacement is most likely in order. It's not too expensive to replace just the motor, quite a bit more to replace the motor/squirrel fan assembly. But to replace just the motor you have to carefully press/pull off the plastic squirrel fans, marking their alignment to each other so they will stay balanced. Then press them onto the new motor.

I found the result to be very satisfying - smooth fan operation with no more squealing.

Once you get the wiper assembly removed, there is an air box below it. When you get the air box opened up the motor/fan assembly is right there. Actually not a bad job at all except for determining how to exchange the squirrel fans onto the new motor.

Ken300D

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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles