1989 Ford Crown Vic locks, seat belts, and corrosion?

Okay I just got a 1989 Ford Crown Vic she is nearly perfect except the lock on the driver side doesn't work at all automatically or manually and the lock that I'd push to lock it from the inside feels very loose and the key won't turn in the door

The seat belt are very loose they don't retract hardly any

3 there's alot of corrosion on the battery now I heard the coke thing poorig it on helps but how long do you keep it on and would Dr. Thunder (a generic Dr. Pepper) work just as well?

If someone could help me figure out how to fix these things that would be great my Dad said its an easy fix but he didn't tell me how to do it so if someone could help me that would be great

Added (1). And now the battery is dead and jumping isn't helping

You'll have to remove the inner door panel & look at the lock linkage. There were a couple of plastic connectors used - One of them could have snapped. The seat belt springs do wear-out. I would think that belts from a Mustang would fit just fine, so look-up a place that sells Mustang restoration parts. They might also have the broken plastic part from the door. Big Fords are famous for corroding the battery cable clamps. Get a tooth-brush sized brass wire brush & scrub the battery clamps. Also get a battery post cleaning wire brush. Take the clamps off the posts & clean both the clamp & the post. Rubbing some grease on the connected clamp will slow the corrosion, but not stop it. There are felt 'washers' available with a chemical on them that you put over the battery post before you install the clamp that slows the corrosion. Keep that little battery post cleaning wire brush handy, because you'll probably need it once a year or so, especially if you live where it's humid.