I noticed i'm getting moisture coming inside the door and soaking into my door panels. I know modern cars have a piece of plastic glued to the door between under the door panel. Does any one know what kind of glue that is? Could I use some kitchen caulking and some thick plastic?

 

Butyl tape should work good for that. Holds on pretty good but can still be peeled up. Needs some clean up tho

 

What type of clean up? And that butyl tape comes in 1/4 of an inch.

 

If you leave it on for a while a then try to pull it off it leaves residue behind.and you can mold the tape to be whatever thickness you need

 

So what type of plastic can I use to cover the whole? I also want to cover the speaker that will be in the door.

 

I've heard of people using a shower curtain liner. I would think any heavy plastic would work.

 

If the moisture is just on the lower section of the door panel you might want to make sure the drain holes at the bottom of the door are not plugged. The factory holes are small and plug easily.
I ran a 1/4” drill bit in 3 separate locations along the bottom of my doors. The plastic barrier behind the door panel is primarily for air infiltration rather than waterproofing.