[selmeralto]

A last resort, if you can't get the color coding: you should be able to test this by ear.

Put on a track or two with tight bass lines. Face the rear speakers, sitting in the center of the car. Move your head back and forth slowly. Listen for tight bass and clear location of instruments. Reverse the speaker connections on one speaker only. One of the connection setups should have tighter bass with a firmer sense of the location of instruments. Those two speakers should then be in phase. You might find this test easier by setting the fader control to shut off the front speakers.

Then, setting the fader control back to normal, listen to the whole system for the same parameters, paying attention to spatial location and tightness throughout the system. Then reverse *both* connections in the back, keeping them in phase with each other, and listen for consistent phasing throughout the system. The goal is to have a clean, firm, consistent and spatially defined sound throughout the car, at least as far as that's possible in a small moving vehicle.