trailer wiring, 6 wires-->7 pole
 

2003 Chevy Blazer. Never had tow package installed until now. I found the wiring package still intact at the rear and after opening it up a bit, i discovered only 6 wires, not 7. I bought a 7 RV blade set connector. My question is why is there no 7th wire (the black one) and can I still connect 6 wires to the 7 pole?
thx for any insight (I am new to travel trailers and 6,7 pole trailer wiring)

I suspect the Blazer would not have previsions for a back up light power in its trailer harness

this site has how to videos and wiring diagrams
http://www.etrailer.com/