How do you fix a stuck or locked rear seat belt in a 1999 buick? if it cannot be fixed how do you replace it?

The SAFEST thing to do here is to just REPLACE them! Depending on how severe the 'Accident' was you may not be able to repair it. When triggered the mechanical parts the seat belt work on 'Lock' and on most models cannot be released.

You can't it is not designed that way. The seatbelt mechanism is damaged and needs to be replaced.

You do not fix seat belt retractors, you ALWAYS REPLACE THEM.

The only thing a Dealer will do is replace the entire seat belt so I suggest you unbolt it and buy a new one then Fit it yourself

The problem:
The set belt stop has been lost so the seat belt retracts too far when the seat is laid flat. When laid flat the release mechanism locks as it should when not level so the seat cannot be raised up as the belt is too tight and locked.
The Solution:
1) Push the two little clips to release and remove the headrests.
Pull the grey lever on the right of the seat base, lift seat base up and forward toward the front seat..
2) See two red capped leversat the back of the two seat hinges and lever them to the upward position, this releases the seat.
Rock the very heavy seat unit upwards and lift complete unit upwards.
3) Take it out of the car and turn 180% so that the back of the seat and base are in the upward position and the seat belt will pull out as it should
4) Replace the missing belt locking stop (I used a tie pin permanently fixed using super glue).
5) Fold seat back down and test then either re-fit seat unit or leave out to decrease fuel consumption..

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.
  

you may have to remove the bolt that holds the buckle to the floor. Try to push the belt into the retractor and see if it frees up.

I assume you are saying it is in the locked mode. It has an automatic lock mode for holding a child seat. When the belt is pulled all the way out, it goes into locking mode. You will hear it ratchet when the belt starts going back in.



Section 01-20A: Restraints, Active 1999 Taurus, Sable Workshop Manual
DESCRIPTION AND OPERATION Safety Belt, Lap/Shoulder The dual locking mode retractor on the shoulder belt portion of the combination lap/shoulder safety belt for the front seat passenger and rear seat outboard passengers operates in two ways:

1. In the vehicle sensitive (emergency locking) mode, the shoulder belt retractor will allow the occupant freedom of movement, locking tight only on hard braking, hard cornering or impacts of approximately 8 km/h (5 mph) or more. The front and rear outboard seat belt retractors can also be made to lock by pulling/jerking on the belt.
   

1. In the automatic locking mode, the shoulder belt retractor will be automatically locked and remain locked when the combination lap/shoulder safety belt is buckled and does not allow the occupant freedom of movement. This mode provides tight lap/shoulder belt fit on the occupant and on a child safety seat or infant carrier installation restraint.
   

1. When the combination lap/shoulder belt is unbuckled and allowed to retract completely, the retractor will switch to the vehicle sensitive (emergency) locking mode.
   

WARNING: REAR-FACING INFANT SEATS SHOULD NEVER BE PLACED IN THE FRONT SEATS.
The automatic locking mode must be used when installing a child safety seat on the front passenger seat and rear outboard seats where dual locking retractors are provided

You pop the bottom seat cushion first, some models depending on the trim level used a bolt on each side. Lift up the cushion and you will probably see everything you need to remove the belt, If you still need to remove it, there are 4 blots at the bottm of the backrest of the seat.

you can remove the bolt that holds the seatbelt to the floor by pulling off the seatpad where the belt goes to the floor and then raise the seat and the belt should reset. this happens on my passenger side rear seat.

try parking on a level surface, then gently feed the belt back where it comes from, very gently pull it back out as if you were putting it on. hope this helps