This is a 1991 Nissan D21 2WD manual transmission truck. Recently I noticed that the taillights and the brake lights became inoperable. When the brakes are applied, the marker lights in the front light up and the dash lights up as well. This happens even when the key is out of the ignition and the vehicle is turned off. The fuses are good and the bulbs in the taillights are good as well.

A little history. A few months ago a friend of mine borrowed the truck and left the window open during a thunderstorm. The entire interior became soaked and shortly thereafter the automatic turn-off for the turn signal stopped working. Meaning I have to manually turn off the turn signals instead of it occurring automatically. Still clicks, just doesn't turn off. Everything else within the steering column appears to be in working order.

As mentioned, truck has manual transmission and also has manual steering, no A/C and no power windows or locks. Everything is basic.

Frankly, I'm puzzled as to what is occurring here. Any help would be appreciated. Thanks in advance.

Festoids

Obvious answer is to take it to a good mechanic. Obviously the electronics got messed up, and you probably don't know the full story of what your buddy did to the truck o_o. Or if you want to and the problems cost too much trade in your truck for another one. Midsize pick ups are great and if your looking to upgrade a Ford Ranger is a great dependable little pickup truck and is the only truly mid siz pickup left, and takes a lot of crap thrown at it. Also it can be very affordable or you can go for one of those durable old school tacomas or hilux, or for a frontier.

Quote"The fuses are good and the bulbs in the taillights are good as well."

The very first thing I'd check is those bulbs again. Be 100% certain that a double filament bulb (1157) is not in a single filament socket. Or what's worse, the direct opposite.
That would cause a bridge.
I'm also betting you have some ground switching going on from the water damage. Look for any green corrosion in the light switch or behind the drivers lower kick panel area.
Take connectors apart and physically look at them.

Lending my things out has been a learning experience for me too. Sometimes it just don't pay to be nice!
I also agree on the older Tacoma if you want to replace this truck. Yes, they're not cheap. And one reason why, they're that good!
Good luck.