To add some visuals to what Av8r3400 said. Your starting point should be the 5 pin flat plug that every Volvo has close to the rear brace containing the bobtail lights. When you cut the tie wraps you'll notice that it has good length to move it anywhere, it's resting on the frame.
The five wires going to it are: Ground, Reverse, Running lights, Left and Stop, Right and Stop.
You can see new wires and plug going to it the start of the new harness. first going to the three center lights,
and then to the mud flaps lights.
I have a kit of the parts that go with the Volvo plug, they are available from Waytek