There are a couple of things you will need in order to add a 7-way trailer connector to your 2002 Freightliner, starting with a 4-way you will need to hardwire in, the ZCI Circuit Protected Vehicle Wiring Harness with Installation Kit # 119250KIT.
This does come with a converter which allows it to be used with your 3-wire system so you're good to go there. The other part you will need to complete your set up is the Universal Installation Kit for Trailer Brake Controller - 7-Way RV and... view full answer...