The problem is the signal goes to the speaker, and like older BMWs with no amp, has an inline crossover that 'filters' the signal.

In those cars running fresh wires from your new aftermarket amp to the new speakers(or crossovers if you have components) is going to be your best bet.

Euro vehicles have just a radio/tape/CD with internal amp that powers a 2 way front system(sometimes coaxials depending on the chassis) and a rear coaxial/full range driver pair.

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2009 E70 X5 - Third Row - Rear DVD
Past - 04 4.4i Sport Package