Just a few points to consider:

Note that it is not always easy to reach the floor if you are belted in.
Your harness cutter needs to be reachable with either hand while in harness upside right or upside down.

This is one of the problems with knives.
Knives are usually in a pocket and often they are unreachable if a person is stuck in a harness or injured on one side. Being upside down makes it even worse.

This is also a problem with hanging tools on a keyring.
If the keys are in the ignition of a normal car or truck they are pretty much only in reach of your right hand.
If you are upside down and half resting against the roof they might be out of reach of your right hand too.
and nothing guarantees your right hand is working.

If You want to know where to locate a harness cutter for You then get into Your car, belt up and see where you can reach easy with both hands.
Then try to imagine being upside down and where you can still reach.
Some people get a more than one and put them more than one location.