I don't have an xc90 so I can't say for certain but I know on many Volvos (including my s60) there is only have a plastic plug on the passenger side tail light assembly that needs to be removed to install the bulb. The electrical panel should have an empty socket that will activate with the existing switch.
Other models lack a plug and require you to actually drill out the opposite tail light housings. On these vehicles the switch may or may not be wired to activate the bulb. If it does not then the easiest way to wire it up is to tap into the existing rear fog light and run wires to power the other socket.
A word of caution, manufactures only equip vehicles with a single rear fog light as a safety measure to ensure your brake lights aren't drowned out by the bright fog lights. So while aesthetically unappealing you may be making more of a hazard out of your vehicle by installing a second rear fog light. You may notice that there are some older vehicles were equipped with two lights but all have since changed to using a single light.