Wire Harness...

I have a 2000 Buick Lesabe Custom. The Cat overheated and melted the wiring harness in the truck. Wires located in the harness coming from the cabin area by the battery to the trunk and down to the wheel well. I cut and grabbed another set from the junk yard, and matched everything up based on the colors. However, there are multiple "black wires" and for the most part I matched those up by the gauge (thickness) of the wire. My problem is the original wires coming from the car has one extra wire than the donor car I pulled the other wires from. I assume all the black wires are grounds. If this is the case, can I just tie the extra wire into another black wire? Also, does anyone have a wiring diagram of the wires in the trunk please?

Black and black/white wires are often grounds. If you have a FSM and wiring diagrams for the car you can usually trace the pin assignments for every connector by position. That might reveal whether or not you need that extra wire.