Lighting voltage

[noble1978]

Ive been fault finding on a battery drainage problem. I've found that my linterior ighting circuit has 12v one side and 10.5 the other side when in the central position. I understand this 10.5 is coming from the lucas alarm module but can't find out if it's actually supposed to be that way. I don't think it should but.....

Anybody with any clue on here?