As for voltage if the battery is fully charged and "good" then it shouldn't drop below 12.2 volts with the interior lights on. Resting voltage (no lights or anything else on) should be 12.5 to 12.6 volts on a fully charged battery.

13.8 running is normal. 13.6 usually means a weak battery being charged and will climb up to 13.8 or higher once it charges up. If the battery isn't bad.

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Aftermarket E53 Radio Install