I've recently found that heater setting 3 wasn't working and traced the problem to the resistor that is located in the left hand footwell. It had burnt out.

I've fitted the new unit but am now confused by the new wiring.

The old wiring was

black/black + white/black + yellow/black + red

The new wiring is

black/black + white/black + yellow/ black + blue

I'm now assuming that red has now been changed to blue?

I was going to splice the new wires onto the old wires

If you could help that would be great.