Connect additional 6X9" speakers on parcel shelf Toyota yaris 2002

[Astra7]

I Installed two 6X9" Speakers on my Toyota yaris 2002 1.0  it came with 2  dash and  2 door Speakers.

I connect the new 2 parcel shelf speakers  by splicing  the front  dash Speakers wires  because the head unit is 4 *45 W output, it works fine but the front dash speakers is louder so it's hard to sense the new parcel speakers while driving.

Could  you please tell me the best way to connect the new parcel shelf speakers with out using additional amplifier to get a best out put sound.

Thanks