Set Appliance State:

Isolate Mains Power to appliance as per TOPs P4
Gain access to PCB (use Insulation Mat as required)
Disconnect low voltage connectors at bottom left of PCB
marked CN9, CN10, and CN11 from the PCB and make safe
WARNING! - Mains voltage is expected during test follow
TOPs P4 live testing procedure
Mains Power On

Observation:

Does the LEDs now illuminate? (Fan may still run).
If the leds illuminate:
Isolate Mains Power to appliance as per TOPs P4
Reconnect one of the connectors.
Turn mains power back on.
Repeat (isolating each time) to determine which connector stops the LEDs working.