Hi

Last time I was out on my txt 300 noticed some smoke coming from below tank, had a look and most wires for the fan were melted and charred. On Futher inspection I saw the thermostat module/rectifier/ regulator (saw it called a few things had basically melted . Are there any specific causes for this or should I just start replacing parts. Thanks in advance just looking for some knowledge.

4 wires and connected to the DC fan motor would make it a rectifier.  Overheating of that component would indicate too much current input, too much current draw, or a short circuit somewhere.  You need to test everything from the alternator to the fan and thermostat and lights.

….  Your alternator outputs AC power and this is the device that converts AC power to DC so that DC devices can use it.

Thanks for that, I will start to get everything tested 👍