Went through the same thing. I bought without the trailer package, as I wanted a Class III hitch for piece of mind, accessories, and my 2,400lb camper says so.
I've seen the 4-way wire kits for lights for around $60, they plug into your rear lights.
To use the wire harness at the back you need 3 relays and a 4-way adaptor (the adaptor plugs into the pictured harness and has your trailer plug on the other end). I went to the dealership and checked with the parts department AND my salesman. They wanted $170Cdn for just the relays and similar for the wire harness.

(hope they're looking at the wrong part #)
I went to a nearby Mazda dealership for the relays, $2.80 each. I did'nt ask about the 4-way adaptor. I'm splicing into the harness that is there, colours match up to trailer wiring. Now all I have to do is install a brake controler.
So in Cdn $, I've dished out $370 for the hitch and brake controller, $9.80 for the relays, $20 for the 7 blade harness. Add a few bucks for butt conectors and wire for the brake controller and I'm under the $500 option and have what I really wanted!!

(The agrivation makes the Grande Vitara look really good with it's factory Class III hitch but less HP and only 3000lb towing, I could of gotten them to install a brake controller)