The headrest control solenoid is under the rear seat. Remove the rear seat (push the two red tabs in and lift the front of the seat), it is just inside the LR door.
One line in from the vacuum pump (just inside the RR door) one line out to the headrest interlock (in trunk under headrests). Remove the line that runs up, suck on it with your mouth, it doesn't take much vacuum to get them to drop. If that works, it is in your vacuum pump (do the doors lock/unlock?) or in the wiring or solenoid.

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Gone to the dark side

- Jeff