I just replaced my blower motor and here are some instructions. Total time was about 2 hours.

Steps to replace a blower motor on a 1989 420 SEL.

1. The blower is located above the front passenger foot well
2. Remove the right side panel in the foot well. Lift away from the side and pull toward the rear of the car.
3. Undo all screws holding the panel under the glove box. 6, screws in total, one on the right revealed by the panel removed in step 2, 3 going up vertically under the glove box, one into the side of the transmission hump, and one going into the centre of second panel, which is horizontal and extends to the firewall.
4. Pull on the right side of the panel which had 6 screws, and remove. Remove the second panel too.
5. Look underneath the glove box, this is the blower box. Remove the lower blower box panel but undoing the perimeter screws, and removing the clip in the forward right corner. The plan will fall down revealing the bottom of the blower and motor.
6. Unplug the power cord to the blower at the square connector. One part the square connector is fixed to a panel.
7. Pull the terminals off the motor. On mine, the blue wire was on the left side, and red on the right. My blue wire had a female connector, while the red was male.
8. Undo the Phillips head screws holding the blower. My screw heads were rusted. A little wd40 helped to loosen them. You may want to have one of those tools that can remove stripped screws.
9. Wiggle the blower down and remove to the bench.
10. Remove the retain screw at the back of the motor, holding it the plastic housing.
11. Bend the electrical terminals so that they do not catch on the plastic on the way out.
12. Before removing the motor from the plastic housing, on the bottom the housing mark the motor and the housing alignment. Once the motor comes out, make a mark on the new motor, in the same place as on the old motor, so that the new motor and housing can be aligned. This is important so that the terminals appear in the proper place.
13. Place the unit on 2 - 2”x8” which are edge up, so that you can punch the motor and blower out the plastic house. A little wd40 and rubber mallet come in handy. Just tap the bottom of the motor.
14. To remove the blower off the motor shaft, hang the motor down and prop up the entire unit so it is about an inch or 2 off the work surface. I used some short pieces of 2x4. Make sure the blower fan turbine is supported all around, not just on the sides. The motor shaft will show through the blower fan. Using a punch and hammer, tap the shaft out. You may have to give it numerous taps, I also used wd40 on the shaft.
15. Mark the bottom of the new motor with an alignment mark, in the same position as the old motor.
16. Next, put the fan turbine onto the motor shaft. Mine was a snug fit. I did not use or need JB weld. A rubber mallet was used to tap the turbine onto the shaft, until the shaft and turbine are flush.
17. Install the turbine motor assembly into the plastic case, aligning the alignment marks on the motor and housing. Make sure you’ll have clear access to the terminals. Attach the case to the motor with the screw removed in step 10.
18. Replace the motor blower assembly, screws, panels, in reverse order steps 8 to 2.

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1989 420 SEL

I am having problems with the blower motor on my S420, stays at low position even when it is at high speed. The air is very cold, but seems restricted. I have read a lot of posts on different boards. I was getting ready to order a blower regulator when I stumbled upon a reply about cabin filters. Now I want to check that first, as I have in my younger days spent a lot of money thinking I was smarter than the mechanics, bought parts only to find out it wasn't the problem. Ended up taking it to a mechanic to fix in the end. Had the parts left over for later. Now how do I find the cabin filters as it seems impossible to find a good mercedes repair manual for late models.