If your wiring is normal, your new harness and relay should bypass the existing relay. There are four terminals:

#30 goes to the battery (with inline fuse)

#85 goes to ground

#86 goes to the headlight or parking light circuit

#87 goes to the lights (the harness has two wires connected

to this terminal)

Depending on where the wire from 86 is connected, your foglights will come on when the headlights or parking lights are switched on.

If you want independent control of the foglights (so you can turn them on regardless of whether the headlights are on) I can send you a wiring diagram based on the S70. But you need to have the stock Volvo foglight switch, and the stock wiring in the dash panel, which I'd guess is prewired. Contact me via email for this.