If the brake lights stay on with the ignition off, I doubt whether even a brand new fully charged battery would last more than 48 hours. That's a major electrical fault. You need to fault find the brake light circuit. Has anyone worked on the car that might have inadvertently cross connected a lead somewhere?
Are the brake lights permanently on with the ignition off and when was this first noticed? The resting current drain on the car should be in the 30-50 ma range.
Two brake lights would be a drain of roughly 4000 ma. My 2004 Astra G will go over three months without being used and still start. My 2004 ZT V8, after only three weeks, the battery is flat, so I keep it connected to a CTEK MXS 5.0 as advised by others more knowledgeable than myself on this forum. Many modern cars with aftermarket 'goodies' only last two weeks due to the plethora of computers all monitoring all of the systems. There are many good videos on YouTube under 'finding parasitic drain' which will be of enormous help to you. Meanwhile I'd definitely isolate the battery
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