I just had a bed installed on my 05 longbed. Before the install I had a "Modulite" by Drawtite that operated the trailer lights. All worked great pulling our new Arctic Fox travel trailer. After the flatbed, the modulite caused the trailer to look like the brake lights were on when I engaged either turn signal. Installer never checked the trailer plug for correct operation. I've tried a new modulite, Dodge said there is no flasher unit for turns on this truck, there may be a resistor unit that can be wired in, but I haven't got there yet.

Anybody have a similar problem?

 

The modulite should not affect the brake lights on the truck if it's wired up correctly.
Is the lamp out light in the cluster on?
Does the flat bed have LED lights?

 

Since it worked before the bed install and is screwey now I'd bet that somewhere there is a wire that got pinched/broken and is now shorted to ground or they forgot to hook up a ground somewhere.