I have installed aftermarket fog lights in an S70 using the factory wiring. I have the factory wiring diagram as a JPG file, which may help you. Send an email message to "n0v8or@cox.net" and I'll reply immediately with the drawing image attached.

You do need a relay, and the mounting location is very difficult to access (unless you have small hands or take the instrument panel apart. The drawings show where the relay goes. You can buy the offcial Volvo relay (mucho $$$), or just use an ordinary 12V Bosch automotive relay ($3 at Wall Mart; $6 at the Radio Shack), which is what I used. The benefit of the Volvo relay is that it includes an extra contact to drive a small LED in the factory switch to indicate that the fog lights are actually on. At night, I could see they were on without the extra LED, so did not consider it essential.