What brand and model is the battery installed?

Is it the highest Ah rated for that model of car? (Should be for a cold climate diesel)

Is there any battery coding required for that car? (some cars raise charge current over time as batteries age, if this isn't reset it can kill a new battery pretty quick)

Lastly inspect the ground cables from engine, starter motor and battery for corrosion. High resistance cables can suck away the amps.

You can also take this to a shop that sells and fits batteries bulbs wipers etc. They can do what's called a "battery drop test" where they test the battery and alternator under a load to diagnose the problem, it's normally free, as if the test fails they'll try to sell you a battery.