After 3 or 4 days of rain it finally cleared up today & I was checking out the Mustang.
I had a layer of dew on everything in the trunk. On another couple of occasions I had dew inside the windshield late night, when it was warm that day & temp dropped overnight.

I know my drivers side cowl has a leak & there was a small puddle of water on the rubber foot rest part of the carpet. I am ordering a cowl repair kit & a plexiglass cowl cover. I try to keep my car covered but this ran caught me by surprise. (Cowl & other body workrust repair is2 or 3 years in the future right now, so I need the temporary cowl repair kit).

Would the water from the cowl leak be causing th moisture in the trunk? Or is there probably a trunk area cause. I can't see any trunk leaks. My trunk weaterstriping is about 15 years old but it looks OK, but I'll order that also.

Is this a common 41 year old car problem?

I was thinking about getting a couple of dessiccant bags & putting in the trunk & maybe under the front seats. Anybody on here do that?

I am getting ready to put an amp & sub in my trunk but not until I solve the moisture problem.
Am I on the right track & any advice or other possible causes.

 

Yes, it's not uncommon for the trunk seal to leak. Change it out.

 

Never heard of it,what is a dessiccant bag?

 

The rear window seal could be leaking, or there may be rust pinholes in the sheet metal corners of the rear window channel.
Lay on your back while in the trunk and look up in this area. Look for rust and water paths. look for holes. If you don't see anything use a garden hose and have a friend place a small light stream across the middle of the back glass and look for drips.

You could also run the hose around the outside of the trunk weather seal with the trunk open. Pay particular attention to the area above the tail lights.

If possible leave the vents open and the windows cracked to help it dry out. And lay something over the cowl until your cover arrives.
You're definitely getting water where it really shouldn't be.

 

Dessiccant is a moisture absorber. I used it in my job to keep all of the moisture out of satellite waveguides, it would absorb all the moisture to prevent reflected power in the system and keep a clean signal. The most common application you probably see it in is when you buy new shoes or some electronics. Its that little bag labeled "Do Not Eat."

Art don't even bother with putting dessiccant bags in your car. They would be a one time fix, also they are really not designed for that much moisture. They are meant for humid conditions in sealed enviroments to take care of possible small leaks. Do what James said he's right, replace your trunk seal and go and get your cowl cover.