GMC wiring reverse lights to trailer

[TritonTimTR19]

On my GMC truck,I looked and i have one of those adapters inline that my wiring going to my trailer connects to..On each side of my factory plugs going into the adapter i have 4 wires,yellow,2 greens,and a blue or purple...Coming out it only has 3 goint to trailer harness...I am adding Backup light and I am wondering if the blue or purple wires are for the reverse lights...Can i just modify the plug and add a flat 5 connector an remove the flat 4 wire connector or do I have to buy an entirely new adapter assy?

[pantera]

i beleive the center post on a 7 plug is for your back up lights.