I just viewed my seat

There are two plugs, one for the switches and one for power.

Can't you just reach your left arm in and put a wrench on the bolt by feel?
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks

Can you unplug the plugs under the seat?

I power the 850 seat in my 544 by plugging the control switches in and applying 12 volts directly to the seat.

CAREFULLY TAKE THE SIDE PANEL OFF THE SEAT BASE AND PUT THE SAFETY TOOL OVER THE SIDE AIR BAG SENSOR.

I DID NOT SEE ANY ELECTRICAL CONNECTOR OR WIRES GOING TO THE SIDE AIR BAG, JUST TWO AIR TUBES.

I THINK THAT IT CAN BE TRIGGERED WITHOUT ELECTRICITY!!!

BE CAREFUL WITH THOSE SEATS!0

Once you take the side panel off you may be able to see how to run the seat. I can do it with the seat installed in my 544 by plugging in the side switches and connecting 12 volts.

There are also 4 bolts, one at each corner that are used to adjust the seat up and down about 3". If you remove all 4, you may be able to remove the seat.

you could also take an old short socket of the correct size and grind flats on opposite sides and make an adaptor to use an open-end wrench to do the job. Sort of like many spark plug sockets, but much shorter.

The shortest thing you could fabricate would be to weld a handle on to a 1/2" tall piece of an old socket.

You can do it!
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'96 855R,'64 PV544 driver, '67 P1800 basket case, '95 855, '95 854, the first three are mine, heh, heh, 485,000 miles put on 9 bricks

What you need is a ratchet wrench, the kind that looks like an normal box wrench but with the round end as a ratchet. That should give you clearance.

If your car does not have a powered driver's seat, make sure you follow the wiring enough to unplug it from the harness under the dash. $22 is a good deal.

I am not sure if a wagon booster seat will fit the sedan.

Klaus
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Volvos should be saved from the junk yards, not donated.

Borrow a battery from the boneyard to run the motors.