Low air warning pressure switch

Yeah. Its mind boggling how many wires are in that dash. That manifold seems deep down in there because of the rats nest of yellow wires. On my ‘99 I’ve replaced the low air buzzer switch three times over 25 years. Two of them started leaking air at the metal crimp were it connects to the plastic top. Made the first one easy to find.

 

As long as either the light or buzzer works, you don't need both to pass DOT

 

Yep, i turned my buzzers off on my trucks, nothing more obnoxious after just waking up than 3 minutes of

***bbbbbzzzzzzzzzzzzzzzztttt

 

That pic you sent, with key in on position (but not started), disconnect the wires and then short it with a paperclip or something, check around the cab if nothing changes too obviously doing that.

Might be worth putting 12v straight t your buzzer/light to make sure they do work and its not a fuse somewhere

 

Might be. Both my 99 and 2000 Classics were there, along with the brake switch ( normally open closes at 5 Lbs pressure turns brake lights on.) It’s hard to read them. Ones red ink ones green ink if that helps. If you already have the new one, you’ll see the stamp on it. Since it’s normally closed and opens at 65lbs and shuts the buzzer off. Must be stuck open. Maybe if you unplug it and jump the pigtail it will turn the buzzer on. Then you’ll know .

 

The brake light switch may be on the peddle like an old GM car. It is electric only. Both of ours were. You can here it click on and off with key on and engine off. Press peddle with hand and it should be hearable.
My ‘99 had a recall on it because the clip that held it against the peddle wasn’t strong enough and would cause it to push back and not work lights. Thats when I became a where of that nonsense of a switch on an air brake truck. There are a few things on the Business Class trucks that are not traditional Frieghtliner ways. That series was Mercedes designed and influenced.