I just replace my aux fan on my 91 300E, the right fan would run when I tap on it. I saw the problem after the fan was removed, the fan have sucked a lot of road dirt that may caused the fan to short out. As I already brought a new fan, the old old would be my spare.

Here is what I did, the job takes about 3 hours:

Remove the center tube, where the horns are mounted, three bolts hold the tube. One is top, one on bottom, one behind the radiator. you would need to remove the two radiator clipz, and tile it back to gain access to the bolt.
Remove fan guide. You will need to pull hard, be sure you don't damage the guide or the A/C tube in front of the fan
If you have problem with the right fan, you would have to remove the headlight. But in either case, I suggest to remove both fans to clean out all the dirt behind.

Good luck.

Jack