Manual:
http://www.4kenwood.co.uk/images/mediator/90/Kenwood%20K25CB12%20IB.pdf
Page 34 says: Open the door. If the light does not illuminate, the bulb has
probably blown. Call a qualified technician – do not try to change the bulb yourself.
Chris at eSpares says...
We are sorry but do not advise on repairs in this area as this is very close to the high voltage connections to the magnetron and accidental contact can easily prove fatal - even with the power off!
acp, April 2017