If you've disassembled a p38 Range Rover's HVAC display unit, then please read on:
I just recently took mine apart when I thought the incandescent bulbs were bad since certain button weren't illuminated, but that did not remedy the problem. However, as I examined it, I noticed the clear plastic pieces on the backside of the face panel (between the panel with the buttons and the circuit board behind it) around the fan control ****, which seem to spread light from the incandescent bulb to the surrounding buttons. I then noticed there were no other such pieces of clear plastic around the other incandescent bulbs and spreading out to the other buttons. Is this normal, or should there be clear plastic pieces there as well?

Here's a picture (off rangerovers.net)of what I'm talking about, and it appears to have clear plastic pieces behind the green button backgrounds:

 

put it back together..
remove the rotary **** on the front..(fan cntrl) carefuly pull it off
there is a replaceable bulb you can now replace..