Can s90 seats be fitted to S60

[IMAY2023]

@IMAY2023 If it's not too much trouble and you have a minute, could you describe to me what I need to add for the rear seats and how to lay the wire.

I have seats without heating and with mechanical headrests. New seats with heating and electronic folding headrests.

I have read your 4th message many times and I still can't figure it out.

For heating. Is it necessary to lay some kind of wire from the central tunnel (where the heating buttons are) along the cushion and back of the rear seat?

First things first. 
The rear heated seats are controlled by 2 LIN slave modules (left and right) and they are on the same LIN as the front seat heater modules (left and right).  
Each module receives Power from the rear fuse box and negative from the body goes the backrest heater pad (If I recall correctly).  Check the wiring diagram. https://forums.spaycetech.com/attachment.php?aid=335  Don't worry about the S/V90 series, it's the same for all models and year when it comes to seat climate. 

You did not mentioned if you already have 4 zone climate or just 2 zone. If you have just 2 zone. You need to buy rear seat heat buttons and connect it to the existing free connector but it does not have power and LIN on the other side. (where senter console connects to the car's main wiring. (passenger side behind the cover).  The button shares the LIN with the drive mode switch and the power, I took it from the lighter connector.  If you have 4 zone, you just need to change the digital panel with the rear heated seat version. 

Along the headrests. Did I understand correctly that CEM PIN FL75 and CEM PIN FL76, it is necessary to lay the wire from PIN2 on the connectors of the backrest to the CEM itself, which is on the front panel? Is it possible to somehow replace this and install a beautiful mechanical button on the back of the seat (in my opinion, this is even more convenient than the electronic button on the "TV")?

You can try, all you need negative and positive connected to the backrest connector PIN2 is power and PIN5 is negative.