Trickle charger usually means a small amp, dumb charger, that might kill your battery over time. What you want is a smart battery maintainer that periodically checks your battery for charge, provides a higher charge voltage until it is charged up, and then uses a low float charge of only about 13.4 volts to maintain the full charge. I would hook it to the main battery. Not sure if the brake battery will be maintained or not, but it seems like it should.

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'97 E 300 D