There is quite a bit of baloney floating out there on how you do this and the service manual is only clear after you have done it or seen it done.
Here are the definitive instructions for removing the front seat headrests on Gen 1 (F1) LS400's equipped with power headrests.
First, run the headrest(s) all the way up with the power switch.
Next, run the seats forward all the way and climb in the back seat.
Note that at the bottom of each of the two front seat steel headrest pillars (support posts) there is an about 1.5" wide (roughly square) plastic trim piece with a hole for the steel pillar. There is one for each pillar.
With one hand, push down ( i.e., toward the earth for those of you who are executives) on both of these trim pieces of one seat. You will feel a pair of latches give. You don't have to use a lot of effort, but there is a little required. Then, still pushing down with the first hand, with the other hand, lift the headrest and support pillars up (that's away from the earth for you executive types). Slides out easy.
To put the headrest back, just align with the two holes and push down gently until you feel the mechanism lock.
Ridiculously easy...once you know how.
- GRL