Heated seats

[IMAY2023]

Forget about retrofitting heating elements or the only the control units.  n order to add the heating pads to the seats, you will need to take apart the whole seat, and then wire them to the seats connector.  Those parts are hard to find used and brand-new ones will cost you almost a full used seat. Just look for heated used seats and replace the whole seat.  
By the way, I fitted s90 full lather heated, cooled and massage seats to my s60 and everything works.  Ok, I have the advantage of already having heated seats, I just needed to pull another power wire for massage seats.  Here is the full diagram. 

Who ever is interested I have full set of 2019 s60 momentum black seats for sale in very good condition.  
Front Driver: heated and memory
Passenger: heated electric without memory. 
Rear bench and backrest with push button headrest folding.

[Power6]

Massage seats swap, wow nice!! 

I would go nuts for sport seats with ventilation. Would have to be built up from mixing around some seat parts and maybe some custom work since Volvo has never offered such a thing...

[ana100]

I've found few nice seats from the used car parts dealers, however, most of those have airbags removed. Can I just take the airbags from my old seat and simply plug them, by removing the back cover of the seat?

[IMAY2023]

It's a risk and not sure if there are different variants of airbags according to seat type. 
SRS module is connected to the airbag igniter through the seat harness connector underneath the seat on PINs 7 and 8.  I do not know if the igniter is part of the airbag or airbag is connected to it.  
I would not mess around with it.  just look for intact seats. Besides, you can never be sure if blown airbag caused other damage or not.