Note, on the 3.7's the alternator is controlled via the BCU which is quite a complex bit of electronics (read...expensive) I personally would not add an ammeter or voltmeter into any of these circuits as Roadsters have proven to be generally reliable on electrics and you wouldn't want to add complexity or fc-uk up the circuit boards within the BCU with spikes & backfeeds

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^ This is what lies within the ££ magic box