Of coarse ideally you'll run new thicker wires to your aftermarket speakers from a 4 channel amp. But what if you're not installing an amp, and/or just don't want to bother with or don't feel like you need to run thicker wires? Enter this method
Hack the plug off the old speakers, solder new wires to them to go to the aftermarket speaker, then it's just plug and play
Just make sure to use plenty of heat shrink to insulate every thing at the plug (I use 3 layers since I sandwiched them in with the speaker adapter). Will be doing the rears just like this later.