Just finished replacing the trunk lock assembly - it's in great working shape now!
I wish there were more pictures - but at the same time, the job was a major PITA. I'll try to share what I can here. This is a great way to restore your trunk lock. I'm not sure if the 500E has the same part number as the 300D but my part number was NLA (
124-750-01-91
) and I was able to swap the parts over from the generic assembly.
Here's what you get when you order
124-750-15-91
:

- lock assembly with the brass rod
- generic key (despite providing drivers license + registration; it's not keyed to my car).
Basically, I took the assembly apart. Near the part where the brass rod snaps into, there's a cylindrical piece above it which barely pokes through the back of the assembly. It has two springs on it, one sticking out from each side. You can turn this by hand to line up the slot and push it down, and then pull it out at an angle. The bottom has a straight piece of plastic which is going into the cylinder - so it's a bit tricky to remove.
With that removed, you have access to the plastic part the rod is attached to. This is fastened onto the cylinder using an E clip. Use a flathead screwdriver and you can pop that E clip off.
At this point, you can put the key in and turn it. Find the right angle and the cylinder itself will push out (you can push the end of the cylinder for extra help). The tumblers shouldn't fly out since the key is in place. I did this on both the new assembly and my old assembly. In order to remove the key from the now removed cylinder, you need to push the spring at the end of the cylinder down and simultaneously push the pseudo-tumbler (nearest the key) down. Be careful as you remove the key- the tumblers will fly out if you do it too quickly.
Here's what things look like at this step
Once both keys were removed, I swapped the tumblers. There are 3 full inserts which are the same that I have in my rekey kit (for the center console box; numbered 11 - 15). Then there are 3 sets of 2 halves. As you pull the tumblers out, make sure the spring does NOT come out with it. I stripped the new one and swapped the old parts over into the new one, one at a time. Once you finish that, you can put the key into the new cylinder and verify it fits. When it's loose like this, you'll have to press the tumblers in by hand and pull the key back and forth a bit to get it to slide through. Once the key is in all the way, you can't pull it out (until you press the spring down with that one pseudo-tumbler).
At this point, I realized the housings are not the same and it's not a simple rekey. Basically, the trunk latch itself has three holes in it and the lock assembly has three poles. They line up and help you put the two pieces it together - but it didn't fit (old latch with new housing). If you're up for the challenge, you can use an X-ACTO knife and carve away bits of the new housing until the latch does fit
I opted to take off the rest of the cylinder housing (the metal push button, basically). On the plastic cap that the metal comes out of, you need to make sure the notch is lined up with an area they provide for you to put a screwdriver into (to hold the button in). You can see the notch in the top above the screwdriver. When it's in this position, you can use a flathead screwdriver to pry the small 1mm gap between the bottom housing and this cap.
In the above picture, you can see I got it loose and almost removed but didn't remove the cap yet. Once you do remove the cap, the empty cylinder housing and the spring will come out.
At this point (once you do this for new and old), you have two naked housings. Here are the plastic housings stripped down side by side.
The
LEFT one is
124-750-01-91
(from my 300D)
The
RIGHT one is
124-750-15-91
(new part)



At this point, I swapped the metal parts over. Putting the cylinder housing in is very easy. Clean the old part, lube up the outside of the metal with some gleitpaste, and install into the old housing. Put the cap back on and then you can put the cylinder in place.
Installing the cylinder into the housing is a pain. I bent the spring both times and had to carefully bend the spring back into shape. Basically, the cylinder has a notch in one area. See the below pic with the red arrows which point at where the notch starts and stops.
That portion needs to line up with a raised bevel inside the cylinder housing. Here's what that looks like (this is the old part - which is why it's so dirty). I traced the raised part in red and used an arrow to point at it.
You have to put the spring at the end of the cylinder, press it down, and then insert into this housing. That's the part where I bent the springs. After multiple tries, I got it and the cylinder went right in. You can give the key a few turns, make sure it locks/unlocks and that you can remove the key without an issue.
At the end of the cylinder, you can put the E clip back on, which keeps the cylinder in place. The part with the two springs goes above it (with a long piece going through the end of the cylinder).
I hope some of this is useful. In retrospect, I should have taken more pics. But I can walk anyone through this, it's pretty easy if you take your time. After you get this main part sorted, you install, then snap the latch down on top of it, push the whole assembly in, and then have the fun part of lining that up, making sure the rubber weather seal is in place, etc.
All in, this took 3 hours start to finish. That's with me installing the part multiple times the wrong way, feeling hopeless, cussing, etc. It's working beautifully now! Key goes in buttery smooth and pushing the button itself unlatches the trunk every single time. Before, you'd have to hit it or try like 5 times before the trunk popped.
Total cost was $126. I think it was worth it, given how often you use a trunk. While you can disassemble your unit and clean it, mine actually had a broken part. There's a flap that is on the cylinder itself that keeps the elements out of the cylinder:
That flap is a spring loaded flat piece of metal that swivels as you put the key in. The spring in mine broke which means dirt and debris have been getting in the keyhole since I bought the car. And then to help give the full flavor experience, the faceplate actually fell off the cylinder as I was taking it out. Perfect timing on this replacement!
