Install replacement front door speakers
The new Alpine front door speakers are direct replacement for the old ones.
Front door panel with original mid-range speaker.
Undo 3 small nuts and install a new speaker. Attach a crossover (diplexer) to two of the bolts. One of the blue wires from the crossover connects to the new mid-range speaker. Remaining blue and yellow wires have exactly the same connector. One connects to the original mid range speaker cable coming from the amplifier, other - to a new tweeter. In my case, yellow wire had a label "Radio/Head Unit", but this was incorrect! The yellow wire, labelled as "Head Unit" had to be connected to a tweeter and the blue wire with no label - to the original speaker wire coming from HU/Amplifier. This actually matches colours or the cables (tweeter wire is yellow and HU wire is blue). If you connect these wires wrong way around (as I initially did), there will be almost no sound coming from the mid range speaker and way too much sound coming from the tweeter. Make sure you test before putting everything back together.
New mid-range speaker with the crossover. This photo was taken before I figured out that yellow cable was labelled incorrectly by manufacturer. It makes more sense to install the crossover with yellow (tweeter) cable facing upwards.
Push a tweeter into your new door surround trim. It should go in with a little click and stay there securely.
Don't forget to put a foam insulator cover taken from the old door surround trip.
Now all is left is to put everything back and enjoy much improved sound!