I am surprised your mechanic can't read the computer under the hood. You certainly can. Just pull off the cover on the "A" box and insert the probe into slot 1 and push the black button for less than a second. You will get back a series of 3 flashes of the red light. Push it again to get the next code, etc. until you get the first number again. Then push and hold the button for a 7 count, the light comes on steady, and push and hold for another 7 count, that should delete any stored codes in "A1". Those that are still present are your problem.
I suspect that the PNP switch is giving you this flashing tranny light, so try "rowing" the shifter about 20 times from R to L. That usually moves some of the grease around and gets dirt off the PNP switch. This routine may last a week or a couple of months, plenty of time to replace the PNP - Park, Neutral Position switch. Another indication of a bad PNP is the back up lights not working when in R.
Also check the engine codes in A-2 and A-6.

ABS codes are in A-3. Just read and clear them first. The ABS light will not turn out until you take the car up to 20 mph, so don't get discouraged. Usually, one of the sensors at the wheel gets 'blind' because of the rust on the axle splines has filled in the the grooves which get sensed. Just use a wire brush to clean of the axles and you will be good to go. The ABS module seldom fails.

I looked at your 220!!!!!!!!!!! Totally impressed! Can you come by and work on my 220? :)

Klaus
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Proud owner of a 220S. If I had more room, I would have more Volvos.