Middle rear seat belt locked whilst down - 2007 Mercedes-Benz C230 Sedan

Start with the basics. I've been caught out in the past by something obvious and silly. Re-raise all the rear seats. Look at all the links, mounts and lock points where all the seats lock down. Sometimes it's as simple as something stuck in the latch mechanism. Mine was an old work glove. Are the seat belts lined up correctly as these can get in the way as well.
Try resetting the seats in a different order. Some seat systems need to be positioned in a certain way to line up the lock latches.

You do not fix seat belts you replace them, the chance of the seat belt retractor not functioning correctly and failing to lock the belt in a collision is just not worth saving a few bucks, wouldn't you agree?

You must remove the rear seat bottom and remove the bolt for the seat belt retractor, this will allow you to in jam the seat belt.

Check to be sure it is the right belt and latch combination.
I believe you have 3 seat belts in the rear. The latch for the two side ( door) belts is not the same as the one in the middle.
VERY often, one of the latches will fall down in between the seat rest and the back rest of the seat, often from dis-use.
This happens on all cars.
You should have 3 latches on short straps in the middle of the rear seat.
Be blessed.

On my 2016 Ford Escape the rear passenger seat folded down and would not hold back up the problem was that the seat belt buckle pivoted down below the level of the seat when I try to raise the back of the sheet the seat belt buckle would prevent it it ended up crushing the plastic on the seat belt buckle preventing it from working the hold the seat belt I was able to repair the seat belt buckle with a screwdriver regular Style and a cigarette lighter the soft in the plastic back up so it would stay in position

you need the car on level ground and try to take tension off the side that retracts however you can. hold it there a few seconds and try to slowly move the belt. good luck

take it to your dealer and tell them the problem they should be able to fix/relpace the seat belt

The fix is:

• remove the seat bench,
  
• unscrew the bolt attaching the seat belt to the body,
  
• return the seat back to its upright position and make sure it latches
• reattach the seat belt
• replace the seat bench

To prevent this from happening again:

• Leave the seat belt latch in its receiver when not in use.
  

middle seat belt shouldn't have anything to do with it. The seat locks down for stowage in the rear cargo area. you can pull the release handle on the back of the seat to release it back into the up position.

Hi, sit in middle seat facing forward. If seatbelt is mounted in roof over your right shoulder, the clip on the end of the seatbelt clips in down by your right hip. Then the 'free' clip clips in by your left hip in to one of the two seatbelt clips. If you have done it right you should have a lap/sash configuration with the belt form the roof running over your right shoulder. The roof belt is also 'inertia reel' and behaves the same as all the other belts. To get out of the seat belt undo the 'free' or middle point clip only. When you have finished altogether, press key in to end clip lock and store back in the roof.
If you have problems getting clips in to holders press the release tab on the holder.
Hope that helps.

Paul
Australia