Personally I wouldn't bother with the dash speakers. Too small for any real full range sound. Maybe wire them to a CB radio or BT phone device. lol

If you can fit 6.5" speakers in the door that's the way to go.
Leave the stock wiring alone and rewire with quality speaker wire & RCA twisted cables from HU to amp (keep away from power leads to amp to avoid feedback). Also make sure you have a good metal to metal ground for your (at least 8 ga) wire to amp and capacitor.
And if you're pushing a ported enclosure 12" single or double, get a capacitor for it. 1 farad per 1000 watts.
If you're not pushing more that 1000 watts you could probably do without the cap.

German car audio wiring are completely different animals compared to most cars. That's why I'd leave the stock stuff alone.
Besides they're designed for low wattage 8Ω speakers anyways. Everything now is mostly 4Ωs if not 2.

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