All head unts have built in amps, other wise you'd have line level voltages and you wouldn't hear anything.
The impedance of speaker wire is so small that it wouldn't affect the loudness.
Problem is that the speakers arent matched to the power output of the head unit.
If you have 100 watt rms speakers in the back and your head unit is 30 watts rms, and you have 30 watt rms speakers in the front, then they will be louder than the ones in the back, because the head unit doesnt have the power to produce the same loudness in the 100 watt speakers.
Either get some speaker that are the same power rating as the front ones, or if you need really loud music coming from behind you, find an amp.
You chould check out the manual for the head unit (or search for the information on google) to find out if it has speaker level outputs for front and rear, and what power rating they are, if it has line level outputs for amps. Also if it does have front and rear outputs, there is probably a front/rear fader. You'd want to check that the rears aren't set on a quieter setting. I'd probably do that first before buying an amp or new speakers.