Okay, so I did a whole bunch of little stuff to the "new to me" S90 today. Replaced many bulbs, coolant temp sensor to clear the P0116 code, thermostat while I was in there, etc. Two issues in particular that I tackled I understand are semi-systemic problems as mentioned in the subject line. Got some tips to pass along in case folks run into similar issues:

1) Replacing the bulb in the ambient temperature gauge built into the instrument cluster... there are two ways to do this, either remove the upper dash by locating & popping the numerous torx screws behind the air vents and other locations, or by removing the knee bolster above the pedals (and the sheet metal shock absorbing piece) and getting at the bulb while it's still in place... I chose the latter. Two torx screws to pull off the knee bolster, then two bolts & two nuts to remove the sheet metal piece and you can reach up and feel the bulb screwed into the top of the rectangular box that is the ambient temp gauge (which has the approximate dimensions of a narrow deck of cards). Getting your fingers on it is one thing, twisting hard enough to remove the bulb is another, then twisting the new one in place is still another... here's a tip that will help a LOT. The bulb housing has a little gap at the very top... if you bought a replacement you can see what I'm talking about. Stick a jeweler's flat-head screwdriver through the hole and out the hole on the other side of the housing, then it will twist the old bulb/housing out quite easily. It will insert into the old bulb from the front to the rear of the car and twist counter-clockwise (as though you were looking down through the windshield) 90 degrees so that the butt of the screwdriver is facing out the driver's side door. Move the screwdriver over to the new housing and install in reverse. Would take me 10-15 mins tops now that I know where it is and what to use.

2) These headlight wiper motors seem to get stuck in odd positions, then randomly fix themselves sometime later when you turn the car off and it sits for a bit then you turn it back on again. Mine does it, my Dad's S90 does it, and I've seen at least a couple for sale online with the driver's wipers in odd positions. The motor is failing (probably the bearings or the brushes) and as it fails it requires more current to turn the wipers, this triggers the overcurrent/overtemp protection that is inside the wiper motor housing. Mine was failing on the driver's side, so I removed the airbox and used a torx screwdriver to remove the motor and drop it out from behind the front bumper. Careful prying with multiple screwdrivers popped it open. I should have left the gears as-is, because they are apparently in some sort of "home" position. I pulled everything apart and tomorrow I have to do it all again to reset the gears in the proper position. My solution was this (disclaimer - if you catch your car on fire or otherwise damage it by following my suggestion, I am NOT liable.): Remove the small wafer-like piece that is square and about the thickness and diagonal dimensions of a dime from in between the two copper strips... that is the thermal protection. According to my Dad as I described it to him over the phone, it is a "poly switch" or something to that effect... an object that increases resistance as current increases, which is why as the motor draws more current it slows to a crawl then stops, but is fine a while later. He is an electrical engineer with very strong "kung fu" so I am inclined to believe him. I pulled it out, replaced it with a metal washer of similar size, and put a fuse inline with the yellow lead going to the motor (the one that maintains power until it returns "home.") Although the motor is probably a little tired, it still has enough life left into it to turn the wipers. The fuse should blow when it REALLY bogs down and tell me that it's time to replace the unit. I will have to remember to clear off excessive snow before firing it up, or I'll have to dig under the hood and replace the fuse on that side.

Hope this helps!

-Jon