1998 Volvo lowbeams will not come on. Tried replacing the headlight fuse, light control relay, and the turn signal/highbeam arm....still nothing. Am I missing something somewhere? The parking lights and all tail lights still work, and the highbeams will come on if I pull the arm but won't stay on unless I hold it.

If your high beams come on only when you hold the lever, and you have no headlights otherwise, i would suggest it has to be the dimmer switch that is faulty. First, though, use a test light to see if you have power to the low beams. Just pull the bulb out, and test in the socket. If you have power there, the problem is just the bulbs. No... they didn't both burn out at once, but you don't notice until they are both gone. You meet cars all the time with one headlight. When the second one goes, they over think the problem.
The fact that your dimmer switch has to be held to get any headlights would lead me to believe it's the dimmer switch that is faulty.