UPDATE: IT TURNED OUT THE BATTERY WONT LAST MORE THAN A FEW DAYS, SEE NEWER POST AUG 2015.
Last summer I had the infamous clock mechanism replaced by quartz. When installed I fed it by separate cables from the car battery, something I needed as I have a main breaking switch when I leave the car. The upside with a separate feed is that you dont have to set the clock every time you drive. The potential downside is the introduction of new sources of short circuitry or voltage leaks and things like that. And having had various battery problems since then – albeit now solved – I decided to replace that separate feed with a simple replaceable battery. Found a battery holder on ebay for a few Euros including a 12V A23 battery.
Note that the quartz clock is (…or should be if you decide to convert) electrically isolated from the clock housing. The housing ground need to be intact, otherwise the clock light wouldnt work. Removed the old cables, fitted the holder with cable shoes and some isolation tape to protect it – and then fitted the packet on a ventilation hose with cable ties below the instrument panel. 30 minutes work – sits well and works fine. Have no idea how long the battery will last, but given that a quartz clock use very tiny amount current it will probably last for years.