Ned,
Lights dimming is either a bad battery or you may have a bad ground cable. Check all teh connections as this may be the cause and is a lot cheaper than a new battery.

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'99 S420 - Mommies
'72 280SE 4.5 - looking to breathe life into it
'84 300SD Grey - Sold
'85 300SD Silver - Sold
'78 Ski Nautique