I do have to find these but ….

• you don’t need the mechanical backup if you open the door immediately
• mechanical backup can’t help with unequal pressure while you’re sinking
• who the heck would think of that while underwater in a car full of water?

IMO, the real answers are:

1. Train people to open windows when they go into the water and expect to swim out. There ought to be power long enough to do this
2. Make sure there is a window breaker securely mounted someplace obvious

I don’t know if I’d do any better in a panic, but I really ought to get window breakers for all my family’s cars