BMW's are notorious for failing window regulators.
This thread should help you replace yours when/if it fails.
I purchased part # 51337165596 (passenger side/right side) from my local BMW dealer directly for $162.00 plus tax.
If you need the drivers side/right side, it is 51337165595.
I found them slightly cheaper online, but I wanted to get it fixed asap so I paid a few dollars more.
This is what you get.
This is actually the back view.
The larger rail will be closer to the body of the car once you have the door open. (regardless of which side you're repairing)
Step one. Remove the door panel. Check out this vid for a step by step on removing the door panel.
Start with removing this bolt cover.
Remove the T-20 machine screw.
Using a flat head (be careful not to scratch your fake wood), find the gap under the door handle between the wood and handle.
Twist the flat head to pop the wood cover open a little.
Then use your hand and slowly pull it off.
There are little metal clips that hold the wood cover onto the handle.
If the clips move, replace them for the re-install later.
Pic of handle with wood cover removed. Note clips:
Next, remove two T-20 screws from the inside the handle.
Next, you should pry up the window switch housing FROM THE FRONT!
The back side will not come up.
Remove the window switch and disconnect the electrical connection, by simply pulling it off.
I didnt take a picture of this, but the next step is to use a flat head (covered with a rag) and pry your door panel off.
The door panel is held in place with several small plastic clips.
You really cant hurt them, so just get in there and pop that sucker off.
Dont pull it too far away from the door.
Once you get it off you will see the speaker connections.
Just pull them straight off. YELLOW ON BOTTOM.
Next you will need to remove the door handle lever from the back side of the door panel.
I dont have a pic, but just find the white clip, pull the little lever up, and lift the assembly away from the door.
Once you have the lever, speaker, and switch disconnected, it should look like this.
Using a razor blade, cut the sticky black glue adhesive away from the door frame.
Try not to cut the foam, or scratch the paint.
Once you have it completely removed it should look like this.
Now you will have the access you need to pull the regulator assembly out.
Now its time for a very important step.
I'd like to thank Magiik for his sacrifice.
He went through hell to learn a valuable lesson for the rest of us.
Take a minute to read this thread if you are having issues with the next step.
http://www.1addicts.com/forums/showthread.php?t=761814
What you need to know, is that the glass is held onto the assembly by two large bolts that look like this...
Minus my allen key.
Notice the bolt has a T-20 star head fitting on the end of the bolt.
You don't want to attempt removing it by using that fitting.
You will lose.
Look very closely at this pic, and notice my allen key is inside the strange bolt with the gear looking head.
This is how you should remove this bolt.
You may need to use the drivers side window switch to raise/lower the window in order to fit your hand inside the door frame to get leverage.
Its tight. Remove both of the strange gear headed bolts (one from each rail).
The glass will still be held in place by a plastic clip, but be careful.
In order to remove the assembly, you must remove the 3 T-25 screws that holds the motor onto the regulator assembly.
Next, remove the 2 10mm nuts holding the rail to the door frame. Bottom nut^^^
and top nut, below.
The other rail assembly is held in place by 3 10mm nuts.
The top one is behind the plastic dust shield.
The lower nuts are underneath the door frame.
Before removing the glass, get to know this clip.
It sits inside the hole in the glass.
When you are ready to lift the glass out, pull the plastic clips away from the glass, and lift the glass.
This pic was taken with the glass already out, just so you can see the hole in the glass and how the clip works.
NOTE: When reinstalling the glass, remember how much play there is between this little clip, and that large hole in the glass.
If you tighten the gear bolt and this is not centered, your window wont close all the way.
Or it will be too far forward, or too far back.
It is important to center this connection, if you dont want water leaking into your car.
At this point you can remove the regulator assembly, and re-install the new one.
The procedure is the exact opposite.
One more thing I will mention is when re-attaching the motor to the regulator assembly, it might be easier to line up the motor gear into the regulator gear BEFORE you tighten down the rail. You will see what I mean.
This is what my old regulator assembly looked like.
When I googled "135i window regulator" I found minimal results, with very little help. Poor Magiik learned a hard lesson for the rest of us, so I figured I'd try to put something together to give other people a little more info on the topic, since it is such a common occurrence. I hope this helps somebody out.
GOOD LUCK!!