According to my 89 OEM manual, you need a special tool (BT-7933), but it gives direction on how to make your own.
You need 1/64" (0.4mm) flexible steel strap. You need to cut it 6'' (152MM) long by 1-1/16" (27mm) wide. (Cut it longer than 6" so you can curl the end and leave at least 6" of tool.) At 3/8'' (10mm) from the bottom, you need to taper the width to 5/8" (16mm), so take 43307/200000" (5.5mm) off each side. I would use the metric measurements.
After you make your tool raise the head restraint to the full up position. Insert the lock release tool down the front of the head restraint post, about 4-1/2" (110mm). Push both the tool and the head restraint downward about 3/8" (10mm) to release the detent. Lift the restraint. Repeat the process to remove the head restraint from the seat back.
To reinstall the restraint, insert post into guide and push down until it is in the full down position. Check to make sure the lock spring is engaged and restraint cannot be removed.
Again this process is for my 89 Brougham and my not be applicable on your 81. They changed the seats in the mid '80's and I'm not sure if the service procedure is the same.