Working on a 2009 4900 western star. When left or right signals are engaged both flashers flash. When headlights are turned on the signals stay solidly lit and don't flash. There are no rear brake lights or rear flashers. Could this be a problem with the box behind the dash or something else.
It just sounds like wiring. Like you have bad grounds or corroded wires. The headlights are back feeding through the turn signals because they are LEDs that do not take much to light. Unplug the leads at the firewall and see what you get there.
