I dd write up in the tech section search on volt meter, you should find it, I cant post a link?
The clock is BMW.
The volt meter was abot £3 from Ebay I did post a link to the seller as its also very accurate, the switch was £1 from Ebay so you can switch it on and off as required.
Just carefully trim the pocket in the clock surround and its easy, just measyre twice cut once
Heres a few pics

Any questions just ask