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If the door is stuck closed because of a latch failure it can be pretty tough to get in and repair it. Unfortunately there isn't a way to simply release the latch in a situation like this.
What you'll have to do is attempt to remove the door panel while the door is closed. Sometimes removing the seat for better access will help. You should be able to get the panel off with the door closed, but I've seen some who have resorted to cutting an access hole in the panel in some cases.
Either way you'll need to get into the door and access the latch. Once there you'll need to dismantle it using whatever means necessary - pliers and screwdrivers for sure, a grinder or cutoff wheel may be helpful also. What you'll need to do is dismantle the latch enough to access the pawl that clamps around the striker and then use a screwdriver to move the pawl and open the door.
Once the door is open then you can replace the latch with a new one.