The switch, called the "range recognition switch" is inside the shifter itself. You'd need to remove the shifter assembly (which is fairly easy) and disassemble the shifter (which is more difficult, and is kind of like a puzzle) to get to it.
I'd hate to have you go through any disassembly just based on what I am guessing at though. It should be diagnosed properly.
The range recognition switch is available as a seperate part though, just so you know. This, as you probably know, is the 722.6 transmission, electronically controlled. There have also been problems with the valve body and a part on the valve body called the "conductor plate" which can cause problems like you're experiencing. But if the interior was really flooded, it could get to that range recognition switch. OR if they tried tidying up the interior and got some kind of cleaner into that switch area it can be a problem. Just a little moisture can do it.

On the PSE pump, you can try taking the PSE out of the foam box and remove the electrical connectors and check for any corrosion, usually on the PSE it will look very green if there is any corrosion.
You'll see these little locking bales that need to be released to remove the connectors on the PSE.

With the back seat bottom cushion removed, it's pretty easy to put your hand under the rear carpet section (like where the rear passengers feet would rest) to see if the under side of the carpetting is wet, another sure sign of a flood damaged car.

Did the seller claim that the car was flood damaged or not, and did you buy from an individual or a company?

Gilly