Over the weekend I had the misfortune of discovering the defect with the flip down rear seat getting stuck because of the center seat belt getting stuck fully retracted. After searching the web I found posts talking about removing the rear of the seat but nothing showing any steps. I'm new on the site so I'm not sure if this is the right spot for this but here's a step by step to fix the problem.
2002 ML500 (Canada)
Rear 3/4 seat stuck in the down position. Pulling release/lock tab does activate the release cable but the seat will not return to the up position. You may find that you can move the seat an inch or so upward but then it stops.
If you check the center seat belt you will see it is very tight. If this is the case then the fix is pretty simple.
Phillips screwdriver (there are two sizes of screw heads, so you may need a smaller head and medium head driver)
Flat head screwdriver (used to help remove the fabric)
Large Torx driver (used to remove the seat belt assembly)
You're going to remove the back of the seat, then remove the seat belt assembly. This will allow you to unwind the seat belt and return the seat to the up position. This doesn't fix the problem going forward. I inserted a plug in the belt to prevent it from over winding the next time I lower the seat.
Remove the screws at the bottom of the seat. There are six small screws around the connectors for the bottom screen slots and offsetting large and small screws across the bottom.
Remove the back trim. The trim sits in a channel around the edge. Its really quite easy to remove and re-install. I pulled the bottom edge out and then using the flat head screwdriver slid it along to remove the trim.
Remove the back panel.
Note: The top of the panel has two plastic screw/snaps that must be pryed out. Be gentle but firm. Again I used the flat head to get under them and slowly pryed them up until they released.
In the picture you can see the two parts at the top.
Remove top seat belt assembly.
Using the torx bit remove the assembly from the back (theres not cheating to get the belt to loosen..you have to remove it to get it working).
Remove the plastic on the passenger side of the seat belt assembly. This will reveal the gears and ball that control the release. Its hard to explain but you'll need to insert a small flat head or small tool to get the gears to release. It'll take some playing, but once you get it you can unwind the seat belt.
After you unwind the seat belt, raise the seat to the up position. At this point I marked the top point of the seat belt where is goes into the top of the seat. I made a small hole in the belt and inserted a stopper so that the seat belt could not retract any further. This should keep it from happening again. Sorry for the picture its blurry but you get the idea.
Reassembly. Reverse of what you've just done. I lowered the seat again for reassembly. Made it easier to work. Everything goes back on really easy. I was worried about the back trim but it was really easy.
I read online that the dealer charges about $900 for this and charges something like 3 hrs. Took me a total of about 30 min to do and that includes running back and forth for tools and taking blurry pictures.
Hope this helps with anyone else running into this issue.