1. I have an 03 3500, 6sp, 4X4, long bed DRW. is the exhaust 3.5 or 4in, I've heard both at different times.
2. I want to connect nerf bar light to my interior lights, when the door opens a light comes on for my wife to find the step. Where is the door switch, what wire is neg and positive. I've used a light test and am still confused.

 

well on my 01 pipe from turbo is 4' than shrink to 3 between resonator and muffler than go back to 4' after...
yeah guys i ditched this crap already and cutted the muffler entrance where its 4' but ill change to an aero soon ill promise,just after the airdog or fass

 

I'm pretty sure that 03's have a 4" exhaust

 

For the light question you might want to check out this thread....https://dodgeforum.com/m_873481/tm.htm

 

The 03's have a 3.5" from the turbo to the muffler, then from the muffler out it's 4". I have an early 04 and it's the same set up as yours.

No idea on the light question though, sorry.