Low air buzzer stays on

Agreed!

 

You must figure out first where the water is leaking in from and stop it !
Cheap fix If its the marker lights I would find the color of Flex Seal you like and buy a small can and paint it on around the lights & rubber gaskets.
Spend a little $$ and take them all off the cab and put some small lights in your visor. No holes in roof = no water getting in.

Now if its the windshield 99% sure its the driver side and when it was replaced it was not with a new OEM glass and rubber gasket from Peterbilt. After market glass I am told is cut just a bit smaller. Some will say I am nuts but I got two aftermarket ones out in the garage slightly used. Use poly urethane glue not silicone
Replace the sensors down by the brake peddle
New relays in the fuse box, when you pull them out and they are corroded cut one open and see why I just replace all of them.
All the automatic fuses too.
A can of electrical contact cleaner and pull each fuse and clean contact.

 

When you pull the kick plates off down around the fuse box and foot valve notice if the floor is wet or a white dried up kind of dust is present.
Good luck Oh and disconnect the batteries so you don't short out that $70 dollar buzzer. Got one of those too.