Not exactly a diy as its not step by step.
In my accord seat heaters did not work when I bought the car. So I wanted to know what could be the problem.
In internet you can find many diy`s how to fix them and most common is broken heater wire in the middle of seat.
In my case to my surprise it was broken wire where it connects to heat switches in front of the seat.
(As I understand these cut power when some temperature is reached).
Almost forgot, first disconnect battery negative terminal!
You can check for continuity between connector pins, back heater is easy, just two wires going there, checked it, mine were fine.
Next is to check continuity in lower part of the seat, and there was none. Following the wires I noticed that in front of the seat wires seem to be broken.
So from the front, just pull back seat cover, then you can see that white heater material, it is glued together, pull it apart, slower and more careful you pull it, then more likely you can glue it back again with old glue.
In the picture you can see the two green wires broken of the switch, so no current flows, seats are not heated up.
Just solder those back together, put all back and enjoy warm booty.
For the second seat I just pulled cover back with seat still in car, little bit harder to solder, but its doable.
Looks like that part of seat is in quite a lot of stress as such strong wires brake over time.
And this is fast and easy fix.