b) "Disconnect the right brake light yellow wire" - when I do that, the right rear parking light will not come on.

Ahah! That one test just nailed it down. It would seeem it's not the bulb sensor.

Your right park light isn't finding ground out through the black wire, but rather finding ground by going through the bulb, turning around and going back up through the brake filament and the yellow wire all the way to the bulb sensor then presumably finding it's way to some sort of ground, like through the left side brake bulb. When the brake lights are activated, all the power goes down the left side as there's no easy path to ground down the right side other than through many extra filaments, including the right front park light.

One way to prove this is with that yellow wire disconnected and the park lights on, connect a long jumper wire to a reliable ground source, like the black wire over in the left tail light. Touch the other end of the jumper to the bulb base contact in the back of the bulb holder on the right side and the park light should now light.

You need to re-check the grounding and contacts at the right tail light assembly, including inside the bulb holder.

One thing occured to me, is this a white plastic bulb holder on the right? The left side uses a special white plastic bulb holder for the two filament brake/park bulb, but the right side uses a black holder.

Something else just occurred to me. Another thing I can think of is that you've got a broken or badly worn socket hole in the back of the tail light assembly and the bulb holder is not staying seated. Actually, now that I think of it I've had such problems at that very same right tail light assembly in one of my 940 wagons a few years back. What I ended up having to do was bend out and slightly twist one or two of the contacts on the tail light so that when I inserted and turned the bulb holder the metal contact on the bulb holder rode under the contact in the tail light assembly and pinched the contacts tight for proper contact. It's not a good solution and can work its way loose again, but it did work.

It's often a bit difficult to properly see what you're doing in that corner of a wagon, especially on a dark wintry day. If you're having problems, may I recommend you remove the tail light assembly with the wiring still connected so you can better see what you're doing. A 10 mm deep socket on a 1/4" nut driver with a short extension makes the job super easy and you're less likely to drop the nuts into a hidden corner. You just need to open up the right side storage well so you can feed the nut driver and extension in through the two holes in the chassis. Worth the price of a 10mm deep 1/4" socket as you'll find 10 mm nuts and bolts hidden all over the car.

Post back and let us know how you make out
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now