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If both front park lights work then fuses 21 & 22 will be okay and you should start at the taillights. If the right front park light is also not working then it's likely fuse 22 (or the contact behind the fuse socket). If power isn't getting to fuse 22 (probe into the top of the fuse) then that takes you back to the headlight switch, which will rarely be the problem.
If both filaments in a brake light (any 1157 bulb) are lit then that means there's a ground problem at the bulb. Power from the brake switch is going through the brake filament and backfeeding out through the parking filament to get to ground at some other part of the park light circuit (typically the front park light since the left/ride sides are separated at the bulb sensor and the license plate lights are on the left side circuit (fuse 21). You can always pull the right front park to see if that now causes the right rear brakelight to no longer work. It's possible the problem is in the bulb itself with two broken filaments welded together so do a bulb swap just to be sure.
If not then double check everything at the rear taillight, like the bulb holder fingers making proper contact with the taillight (the contact strips sometimes get bent)and that the black ground wire is a good connection. In wagons at this point it's best to remove the taillight assembly for full access. In wagons, the ground stud is behind the rear panel and requires removing the panel to open the connector (a non-trivial task), but you can bend the panel out enough to get an open end wrench on it and loosen it a number of turns. If there's visible corrosion then best to remove the panel for proper access and cleaning, otherwise just loosen the nut as far as you can, use contact cleaner, then work some De-Oxit paste around the contact, snugging it back up securely, giving the spade a turn back and forth as you tighten it to scuff the surface.
If everything now works then you're good to go, otherwise it's time to bust out a meter and work back towards the front of the car. Start looking for power on the brown-white wire beginning at the right taillight assembly, then the branch connector behind the left taillight assembly, then at the bulb sensor socket in the relay tray. The Volvo wiring diagrams for the park light circuits are not always correct, with the US/Canada circuits being slightly different from other markets.
If it's not the right rear taillight holder/contacts/ground then odds are high you'll find the problem at the bulb sensor. I know you swapped the bulb sensor relay, but you need to verify it anyway. At the relay tray, pull the bulb sensor and verify continuity between pins 57R and 58R of the sensor for the right taillight. If the circuit is open (or high resistance) then pop open the relay and re-flow the solder connections for that circuit. Otherwise, probe into the tray socket and verify 12 volts is coming in on pin 14 (for 57R). If the problem isn't the bulb sensor then for your symptoms there's possibly tray socket damage and contact issues on the back behind the bulb sensor socket or behind fuse 22.
If you've got a trailer adapter then you need to check things out there as well.
Somewhere in all the above you should find your problem.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now
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