Assuming you do it correctly you should suffer no electrical ill effects....
Do it incorrectly and head unit damage is likely....
If using the 6x9s to replace existing rear speakers, then splice into stock speaker loom,(soldered connection is best)....
If using 6x9s in conjunction with rear speakers, good luck....As you add speakers, overall impedance will drop,(unless connected in series), as impedance drops, demand on the amplifier increases....
Lowering the load, (impedance), an amplifier sees will increase amplifier output, but at the same time amplifier power requirements will increase, as will heat produced by the amplifier, in short, its quite likely the amplifier within the head unit will simply burn out....
Just in case your wondering, FIRE is a possibility....