I just bought a 1998 Tahoe. The vin number build sheet says it has an "8 wire towing harness". Upon inspection I only found 5 wires, brown, yellow, light green, dark green and a large white wire. I am told by one trailer place that it needs another wire to accommodate a brake controller. And another place that it needs modifications for a brake controller but not an additional wire. Why does an "8 wire factory harness" only have 5 wires, and what's the truth about the wires required for a brake controller? Thanks

P.S. I posted this in general tech because it's logical to think that any GM vehicle intended for towing would have the same harness.