2008 Freightliner Century

If it has the J1938 Data Diagnostic Port on the floor near the door, they can get corrosion from rain water and cause funky problems such as yours.

 

Auxillary PDM (fuse box) in driver side B pillar. Screwdriver will let you access swing panel to open. (4) T25 torx screws to remove PDM. Plug in new one. Look inside connector ports of old one, you will see water damage from upper bunk window that leaks down. Less than $100 from Freightliner.

 

Had this problem on my '10 Century. If it's recently rained, you might have water in the box. Pull all the plugs, remove the box and allow it to dry out. Easy jury-rig remedy is to screw a piece of heavy plastic or thin rubber above the box with a bit of silicone caulk as a gasket allowing the leak to miss going through the vent slots.

 

Little door right by your left hip behind the big door you get in and out of.

 

That looks like passenger side. Same place only on driver side.