I finally got the sub installed. That wasn't easy at all. I went through a rubber gasket on the right side to get power from the 12V battery. Unfortunately, the gasket popped out of its insert and I haven't managed to get it back in.
The harness with the wires for the head unit is apparently an item you can not buy adapters for. I ended up using T-connectors as you can see in the picture. I will improve this later on.
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My assumptions on the colors for the speaker wires:
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The sound is really really good now. The sub just adds the missing link to a great sound stage. I'm not done yet though. Next week I will be getting a Pioneer GM D1004 4-channel amplifier. I will also add some more sound dampening to the doors as that bass travels too much inside the plastic.