That unit is very small with every component within 3 feet of the pigtail/power cord.
A electric heater has heat strip/s
a switch and 2 thermal limiting devices, 1 auto resetting, the other not resettable or serviceable by the user, tending to not be an economically viable option.
There are 2 hot legs of power and an earth/ground wire feeding any home or most businesses, every wire in every unit is either 1 of the 2 coming in to the building, Line 1 and Line 2.
Now if you know the sequence of operation and can recognize the components, thus locate them and are familiar with each as to what terminal is what, there is no need for a wiring diagram to tell you that 2 legs of power feed each load, be it high, low voltage, AC or DC polarity.
Every circuit requires 2 sides/legs of power Line 1 and Line 2.
Beginning on Line 1 go through the switch, to the load/device,
and from the device/load we return to the opposite side/leg of power from which we began.
Now if you do not know this and do not recognize the components thus cannot locate them other than tediously following both legs of power all over to where you know it must be anyways! as the unit in no way resembles a wiring diagram I see no use for a wiring diagram for those who know or do nor know the trade.