Hi I have a problem on my mk4 1998 astra (astra G) with the 3 buttons on the heater control unit. The one marked in the picture does not come on when the headlights are turned on. All other dash lights come on, however the light that tells you the button is active comes on just not the illumination. Does anybody have a guide for replacing that bulb? what bulb is it as standard/what to replace with? and what work is involved in fixing it? any pics/tips would be great.