So those black and red wires should represent line and neutral, correct?
Which means you need to figure out which is which.
This is a little bit of a confusing schematic to do that, because the wire colors do some changing.
1. Which color wire that is attached to the heater is switched by the heater pcb?
2. Look at the noise filter where line and neutral come in. What color wire to the left of the noise filter do you think is neutral?