Left Channel coming out Both Speakers suddenly

[MedMantle]

Hey guys, for months now I have been running quite a basic sound system in my car and recently I discovered in some songs that certain "notes" or frequencies were missing.

Specifically in one song there is a section where the guy sings "To the left, to the right". In my headphones on the computer I can clearly hear when he sings "To the left" that is coming strictly from the left speaker with only the echo moving to the right and vice versa... but now in my car it sounds like all the midrange that is supposed to come from the left channel is coming out both sides with the sound that is supposed to come from the right channel completely missing like it suddenly became mono with only the left channel sound being processed through both speakers??

I am so sure that it was never like this and I have literally checked everything I could as far as my lowly knowledge goes.

Alpine Headunit, Tweeters, Crossovers and midranges - rear speakers removed and bypassed for what it's worth.
Locally produced amp and cabling and a Kicker Sub.

If anyone has any idea what I could check.

Connections to amp, crossovers and headunit are rock solid.
All my speakers are definitely working and I have tried and tested all the different settings on my Headunit...