Why early Nissan LEAF PTC heaters fail and is it worth it to replace them.
Short answer is: It fails because of bad design. Is it worth it to replace with a new one or used? NO.
Long answer is: Nissan LEAF made between 2011-2013 used Eberspächer High-Voltage PTC heater, which is fitted under the 12V battery. As you probably already found out, this is most unreliable heater ever used by Nissan. Some of them failed in first few years of usage, some managed to survive till this day. But don't get excited if yours is still working. It will fail sooner or later. Why? Because of faulty design. First lets have a look why they fail. Look at these two pictures below and think what is wrong in them:
So if you looked carefully you probably noticed that almost all terminals that should be soldered to PCB stayed on PTC elements after I lifted PCB. Also color on them tells soldering had failed long time ago and now causing excessive resistance, heat, sparks, electric arcs or no contact at all. This puts a lot of pressure on electronics that eventually fails in quite spectacular way.
Look at the difference between these two terminals under the natural light. Left one was cleaned, and the right one is as it came from the heater:
And this is what happens when electronics can't handle it anymore. Look at them soot marks on PCB, aluminum and plastic casing caused by DC electric arc. Also if you look carefully you will be able to see aluminum melted on one corner. Not to mention exploded transistor. There were some spectacular fireworks going inside the heater
This failed heater is taken from 2011 LEAF with about 60k miles (~100k km) on clock. Sometimes the heater refused to work, this was the first symptom. Giving some random fault codes, but no fault light on a instrument cluster. Eventually it died completely, also blowing the 30A fuse in DC/DC junction block, which is a nightmare to replace. There were 5 or 6 fault codes telling there is no high voltage feed to the heater and each transistor inside, but strangely no fault light on instrument cluster again! After opening it, initial thought was that one or two PTC elements failed inside shorting the transistor, but after doing high voltage resistance test I found out this wasn't the case. So the only reason it fails like this in my opinion is failed soldering on PTC terminals. I have tried to replace faulty components and re-solder all terminals to make it usable again, but this is another story, which I will share a bit later.
So is it worth trying to replace it with a new one or a second hand one? Second hand one is a big NO! Because it's just a matter of time when it will fail again, as they all got same problem. For the new one, I am not sure if Eberspächer is aware of this problem and if they changed design. If they haven't sorted it and make exactly same heaters, then the answer is NO again. You will spend probably about couple thousands euros or pounds and you will be on the same gamble again. Not to mention all the difficulties removing DC/DC junction block to replace it or replace blown fuse inside it. Is there a cheap solution? Yes it is, but it's a bit dirty one
I personally installed a Thermo Top diesel heater and have no regrets by doing this. I use lamp oil (kerosene) with it, which burns very clean, so not much bad emissions created. Also it uses very little of fuel. Will drop the post about it bit later. Another huge advantage of using diesel heater instead of PTC heater is, it won't affect the already small range of LEAF.
Please note that this post is just my opinion, I am not an expert and I can be wrong. So it's only up to you if you want to take my opinion as a truth or you better stick with Nissan dealership fairy tails
I ordered a new Precharge resistor from thunderstruck. As you mentioned they will sell OEM Nissan Leaf Precharge resistor. Hoping to recieve my package after 4 days. 
