Truck Repair: low air warning light

Question
We have a 2007 Frtlnr Columbia CL112, C-13 Cat. The low air warning buzzer and light are on constantly. We replaced both sending units. Also recently (about 6 weeks ago) replaced the instrument cluster. Any suggestions on where to look?

Answer
If both low air sensor were replaced. then i would have to say you have wire problem. I've never tested wiring on those but your either going to complete a ground with air pressure or take away a ground with air pressure. If you have a power probe it would come in handy.
Want to make sure i'm clear the lowe air pressure is mounted on the center lower dash. Will have maybe three or 4 different sensors. 1 should be a stop light switch. If it's located where i'm thinking. It should have individual wires going into the switch if those get switched around it will act up also those switches are specific to their function so make sure they are right part too.