The CCU read out is about 0.5v below what you'll see at the battery or + terminal under the hood. Measure your voltage there.

That said, I think the 90a alternator on the 97 is too small for the application. Keep in mind that at idle the alternator can only output about 1/2 of its rated amps so 45a. Now throw in afterglow which can be be ON up to a couple of minutes after start up (12a x 6) and it'll take a long time for that 90a alternator to recharge the battery. 98 and 99 E300 got 115a and 150a alternator from the factory with a OAP pulley. I'm running a 150a alternator with 14.2v at idle (13.7v on the CCU) even with lights and aux fans running.

Also the slip ring in your alternator could be worn out and ground straps might need cleaning.

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CENSORED due to not family friendly words