Since your car is the 2.5D it will be one of the last Ph1 V70s so it's likely to be OBD compliant which the earlier ones, like mine, were not. So theres a good chance that one of the phone app based monitors may work. It's not something I've any experience with since they won't work with my car.
Failing that, a digital voltmeter connected across the accessory socket and monitored by a willing passenger should do the job.
Martin
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