As has already been said I think it's a bad idea - you definitely risk damaging the power amplifier if you connect more speakers in parallel with the existing ones.
The parcel shelf is (deliberately) made of lightweight material so you will have difficulty securing decent (and therefore heavy) speakers in place. You will also need to make them capable of being easily disconnected unless(unlike me) you never have to remove the parcel shelf to carry large items. My shelf spends much of its time either in the garage or tucked vertically behind the front seats!
Put the speakers in parallel and you halve the impedance. Power amps are designed to work at a given impedance, halve it and you risk blowing the output stage.
Put the speakers in series and you ask the power amplifier to do more work than for which it was designed and you risk blowing the output stage.
Good luck or just tell the missus that there is nowhere safe to fix them.
leave well alone, you want to keep the soundstage up front, you don't drive in the back do you !!
your best bet is to change the front door speakers to a decent set of 2 way components, Just sound deadening the front doors makes a vast difference to the note, you could add aactive sub or amp and a sub,6x9's are just bad I could write endless threads about them
there a few of us that fade 80/20 to front, bass set to 2 and treble set to 1
If you want loud do it properly with a aftermarket stereo with 2-3 amps and a sub
edit. you could run a amp from a standard stereo usingspeaker level to rca adapter but if you care about your audio you will use a proper processor (audio control lc6i, ect) that will clean and level the signal to any amp(s) but for the same money your looking at a new aftermarket stereo (aka head unit)