I hate the fact that my 97 OBS doesn't have map lights. Makes it very hard to see much of anything in the front of the car. How would I go about installing them, assuming I got a set out of a wrecked Subaru? Is it possible?

I added Legacy map lights to my '95 Impreza.
The problem is that on 1995-2001 models, excluding RS, there's no mounting bracket on the header rail. So I had to pull a bracket from a salvage '94 Impreza (it was spot welded) and glue it onto mine (1993-94 all have the bracket, although the lights were only installed together with sunroof). The Legacy light fits the bracket just fine. Of course you can fashion your own bracket, or simply use something to mount the lights on the headliner, they aren't heavy.
Another option would be to glue a standard wedge mount to the windshield
and install a mirror with built-in map lights.

bump, I'd love to know how to wire this up.

retrofit a mercedes rear view mirror, hook up the lights to a switch, and get the power from a 12v accesory line?

retrofit a mercedes rear view mirror, hook up the lights to a switch, and get the power from a 12v accesory line?

...???

Yeah, i've done it to my RS. On Scoobymods there's a tutorial with part numbers. You'll need the light assembly, 2 screws, and the wiring harness from a car that actually had the map lights to begin with. IIRC, it's about $50 worth of parts from SubieGal.

BTW, which it comes time to swap in the new wiring harness, it looks like it's IMPOSSIBLE to get to the plug inside the dash. Take your glove box off and you can reach it no problem.

An easy way to power the map lights is by fishing the wires from the dome light opening.
This is found in factory instructions that come with add-on map light kit for 1995-99
Legacy models (which is what I have).
Re. Mercedes mirror suggestion, it's probably a mirror with built-in map lights on the bottom.
Such mirrors are actually common on GM vehicles, and are also found on some convertibles.

Would I have to buy a complete harness or just those wires?

If you're crafty, you can just use any old wire for the store. It's like eleventybillion times easier to buy the correct harness.

I'll just try to buy map lights that come with wires.