Headunit outputs full range to all speaks

http://www.mhsoft.nl/spk_calc.asp#crossover

http://www.partsexpress.com/pe/pshowdetl.cfm?&DID=7&Partnumber=027-370  cheap but effective.

A 250 uF cap on a 4 ohm woofer will start the 6 db / octave slope at about 160 Hz.  But keep in mind that it is 6 db per octave, so the woofer output is attenuated by 6 decibels at 80 Hz.

https://www.the12volt.com/caraudio/cross.asp#hp shows the hookup.

90 degrees out of phase is an "in-between" problem that can't be fixed by reversing polarity on one of the drivers, as an 180 degree out of  phase situation would be.  The cure is to reposition one of the drivers if this out-of-phase issue is a problem.  It would manifest itself as a "muddying" of the sound.

Phase shift induced by the various orders of crossovers is something I can't explain, because I don't understand it myself very well.  But, suffice to say, if both L and R speakers have the same crossover, they will be identical with no phase shift between one and the other.  Read  2.1 Phase shift vs. time delay in Rod Elliott's site.

The importance of this phase shift phenomemon becomes apparent when designing a two-way or three-way crossover network.  A design program like Speaker Workshop helps me to get the phase rignt in the network even though I"m not sure how...  But anyway, as I understand it, the importance is the phase difference between two or more drivers in the same network (like a component set or simple coaxial)...not the difference between the front pair and rear pair of speakers in an installed system.

Okay, here's an illustration:  say you have a simple pair of coaxials with the 1st order crossover (capacitor) already built in, which helps to block low freqs from the tweeter.  The drivers in that network (the mid and tweeter) are designed to work together with the knowledge that there is a 1st order, 90 degree phase shift, crossover included.  The coaxial will sound just fine if you leave it alone, for what it's worth.  Now, say you want to mess with it...you pull out the cap and replace it with a textbook 2rd order crossover, consisting of a cap and a coil.  The phase difference is 90 degrees per order, so now you have a 180 degree phase shift between the tweeter and the mid, but you haven't made any other changes to the network to compensate for that difference.  The result would probably be less desirable than if you just left it alone to begin with.

(...somebody correct me if I'm whacked out on this one...)