Working Out Sub Phase and Position?

Hi guys I was wondering how you guys work out which phase a sub should be in, in accordance to the rear speakers (or any others for that matter). I have my sub in the boot (or trunk as you yanks would call it) facing the rear (low-passed of course), and my 6x9's in the rear panels facing upwards (hi-passed). I cant really tell the difference most of the time in the bass level when wired either way (0 or 180 out). All i know is that the 6x9's always seem to overpower the bass when turned up, no matter how they're wired. Any ideas on what to do? we'll start with this problem, and not through in the front door speakers which are mounted facing each other, and my tweeters on my dash pointing towards the center of the car

Also, on a different note, my sub amp (600w, 2 channel) seems to only except a signal from the left RCA input when bridged, is that normal?

Yes it is normal for the amp to only accept a signal from the left channel when bridged. By the sounds of it the front speakers are OK and the rear speakers are starting to break up when the volume is turned up. Are the backside of the rear speakers open to the boot? If they are then the output of the subs will be causing the 6x9" speaker cones to move, you need a gadget called a hat or baffle made by companied like XTC or American International. Next how are the rear speakers being operated, amplifed or deck power? as well as the front speakers?Top Secret, I can tell you but then my wife will kill me.

Sorry for the late reply getting back to you.... The speaker are indeed open to the boot, but it did this in my old car too, and they wasn't. The rear speakers (6x9's) are amplified with a 300w 2ch amplifier. I have had to set the gain to nothing, as if they are any louder, although they still dont clip, it just drowns out the sub (no bass in car). Oh, and to clarify, i only have a single sub, if that makes a difference. Why would it only accept a signal from the left channel? it doesn't make much sence, as a bass note could come from the right channel too...?

Oh, and the amp has a bridged switch.... which is obviously on...