I doubt that anyone has to play with the wires on the bridge 15 and bridge 5... 15 should control leds of wiper intermittent control and spoiler buttons and bridge 5 should control leds of central locking, rear window defogger,and PSM buttons.... But why the PSM completely goes off with engine off and ignition on (post#6) ??? I just went in my car to play with the dimmer and the radio lighting is also dimming with the switch, does it work properly on your car Ant?
note: if you are looking for the wiring diagram for the rear window defogger button it is part of "Mirror adjustment" section.

 

My car is fitted with PCM1
It dims as intended when operating the dimmer control

 

Having proved out the wiring of the illumination LED in the PSM switch there is only one possibility left: the LED is getting reverse polarity. There are 4 ways I can see this happening:
1) The LED wires are swapped in the PSM switch. This cannot be as the PSM switch works to switch PSM on and off as verified on the cluster (correct?). Just in case, pin A4 should be red/blue, pin A5 should be gray/blue/brown. I believe the pins are labeled on the socket.
2) The LED dimmer has been swapped between the PSM and Spolier switches. Seems unlikely. If the gray/blue/brown wire is in place on the PSM switch socket it is correct.
3) The gray/blue/brown wire from the PSM switch has been swapped into the wrong junction behind the relay panel. This seems quite unlikely.
4) The LED in the PSM switch has been removed and reinstalled wrongly, or was manufactured wrongly. This can be tested by removing the PSM switch and applying 12V to terminal A4 and ground to terminal A3 which ought to light the illumination LED at full brightness.

Also having proved out the immer circuit for the spoiler switch by checking the heated seat switches there are only five reasons the spoiler switch will not illuminate:
1) The socket for the spoiler switch is wired wrongly. Pin A3 should be gray/blue/red, pin A4 and A5 should be brown.
2) The gray/blue/red wire has been swapped with the PSM switch.
3) The gray /blue/red wire is not connected to the junction behind the relay panel, which also seems unlikely.
4) The ground at the spoiler switch is faulty. Does the switch work the spoiler? If so then the ground is fine.
5) The two LEDs in the spoiler switch are installed reversed polarity or are defective. Check this by removing the spoiler switch and applying 12V to terminal A3 and ground to terminal A5.

If you have a 12V LED test lamp that is polarized (ie, won't light up with reversed polarity), or you have a plain LED and a 4.7k resistor you can test the car side wiring as follows:
Spoiler switch socket, LED "+" to pin A3 (gray/blu/red) and LED "-" to A5 (brown).
PSM switch socket, LED "+" to pin A4 (red/blue) and LED "-" to A3 (gray/blue/brown).

In each case the LED should light with the other dash LED's and dim properly.
DON'T use an incandescent test light as that might damage the cluster's dimming circuits.

 

that contributory post had to be over book time. great post, please reply more often.