Don't own an S80, so can't really say about your car, but every Volvo I've owned had 2 holes in the front (plastic) bumper. These aren't just any holes, they have something embedded in them with threads to screw in either a bracket or a license tag directly into them (like I did with my wife's V70 and euro license plate which I drilled 2 holes into). I would try autozone Pepboys or other autoparts stores with your dilemma. Maybe they have some kind of "counter sinking" screws that can be inserted into a drilled hole. Just make sure whatever you insert into the bumper is the right size for the bracket you were sent, and you measure carefully before drilling. Other than this, it's off to the dealer for you. Before asking questions, I would walk up and down their car lot and look at other S80's and see how their bumpers are constructed, and ask the question as a last resort. Don't be ashamed to go to the dealer about it.

I have to go into the dealer this morning to have a fan/blower motor noise fixed on the still under warranty V70. While I'm there, I'll take a look around at their S80's and ask some questions about the bumper holes for you. It will be in the afternoon that I can get back to a computer to post my findings, but I'll be back.
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1998 S70 T5 Emarald Green Metallic, 2004 V70 2.5T Ruby Red, Previous Owner of Black '94 850 Turbo Wagon. My cars have been running so well lately they've got me worried.