I have searched the forums and read the stickies but couldn't find any information about installing the wires for aftermarket speakers in the doors.
Must new wires be used or i can just use the existing ones? Do the harness plugs fit aftermarket door speakers?

 

You can use the existing wires.

The harnesses wil not fit.

edit: If you do not know what you are doing, spend the money to have someone do it for you. Less headaches in the long run.

 

You can use the stock wiring, or you can run it thru the stock rubber door boot. That's generally how I run wires.

 

Monty, 5th gen doors have harnesses in the door boot. You cannot run speaker wire through it. Theres a small grommet, about half inch, near where the door boot meets the car. You can run it through there, or use factory wire. In most cases, factory wire is just fine, and will save you a ton of headache.

 

I've run them through there before, there's usually an empty space in the plug, where there are no prongs. You drill thru there. However, stock wiring is far easier.