I need to store my 99 E320 for about eight weeks. Should I disconnect the battery, or will it maintain a charge for this length of time?

Whether it will last may depend upon the condition of the battery itself. The safest thing to do would be disconnect it and attach a trickle charger on it to maintain a charge while stored. Sitting in a state of low or complete discharge, connected or disconnected, will ruin the battery.

Good luck.

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Ted
1979 240D
160,000 miles