trailer wiring

[Ggg]

I am trying to put trailer plug wiring on a D350 DRW box truck (picture a U-Haul type of truck). I have read on here that the stock electrical wiring gage is minimal at best, and not to tap into it for fear of overloading the stock wiring.

How do I tell if this truck has a trailer wiring package or heavy duty wiring?

What would be the best way of wiring into the truck for trailer lights and brakes?

Thanks.