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Had this problem with old 24v Cabstar. In the end I ran 2 x 12v batteries in series as the main 24v supply and tapped into the cable between the batteries to get 12v. Used a 24v relay in the accessories circuit to switch the 12v supply to the stereo and lighter.
Maybe you could change 2nd battery to a pair of 12v. Also 12v batteries are cheaper and easier to get than 24v.