RE: Warning Chime with turn signal
If the dealer INSTALLED the flatbed, as you stated, it's their problem. Just because you chose to have a flatbed put on by them, doesn't relieve them of the responsibility of a proper and correct install.
Take it back.
It sounds like a power wire backfeeding into the system, due to improper installtion somewhere. Aftermarket bodies should also have a separate ground to the frame, and not reply on the metal to metal contact of the body and the frame or U-bolts that hold the body on.
A circuit searching for a ground can cause funky issues as well.
2002 Dodge Ram QC, 1500 4x4
Borla cat-back Exhaust
Full Synthetic oil
Pine tree air freshener-Green
All mods have been thoroughly tested and approved with the "Ass-Dyno 1000i"
Don't be fooled by immitations.