I'm wondering if it has had a low air warning switch fitted in place of the stop light switch? If so, the light will be on until the hand controlled is pulled down.
The low air switch should be at, or in the feed line from the air reservoir. There should be a brake light (brakes on) switch in the control line to the trailer.
Maybe just make sure/test that the right type of switch is fitted, i think the low air switch would be a normally closed switch, and the brake light switch would be a normally open switch.