All, thanks for the advice. Here's an update: I had both the battery and alternator tested and it did in fact turn out to be a dead battery. It's intertesting how they can just fail instantly with little warning. Anyhow, I opted for the OEM battery for $100 and all is good to go.

Cheers.

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2004 E320 Wagon
1989 300SE
1998 Range Rover 4.0 SE