Ahh, i see what you mean now - I didn't have that problem as I'm using a cross over and had to cut the white connector off and attached the wires directly to the unit. If i wasn't though I would have still done the same thing but used a small terminal block instead of the cross over.
Personally I wouldn't worry about the harness / connector but I suppose it does make it all nice and factory fit style.
In regards to speaker size the only unit you are going to get to fit directly in is a 8"/20cm - these should just screw straight in with no problems. With the 6 1/2" you are either going to have to modify the existing adapter the stock speakers are mounted in, buy a ready made adapter or make your own from a sheet of MDF.
Personally I wouldn't worry about the harness / connector but I suppose it does make it all nice and factory fit style.
In regards to speaker size the only unit you are going to get to fit directly in is a 8"/20cm - these should just screw straight in with no problems. With the 6 1/2" you are either going to have to modify the existing adapter the stock speakers are mounted in, buy a ready made adapter or make your own from a sheet of MDF.