Your 2001 International 4300 box truck should have a combined taillight wiring system, which means that the turn signals and brake lamps are combined on the same wire, just like most trailers are wired.
For a 7-way trailer connector, I'd recommend using part # 20023 which would wire up as follows:
• Black---12V power from battery
• White--ground to frame
• Red---left turn and stop tapped into truck's taillight wiring harness
• Brown--Tail and running lights tapped into trucks taillight... view full answer...