What is this? and rear sound problem

[eddiexplorer]

Hi, im knew here and just got my first explorer ever.

I dont get any sound from my left rear speaker, any thoughts or troubleshooting tips?

Anyway I removed the dashthing today to see if any cables were loose and i found a cable socket that wasnt connected to anything, what is that cable doing?

I was also thinking of removing the RCA cables from the rear DVD and add to a 3.5mm jack coming out from the empty 4*4 slots in the dash since I dont have anything there, is it the same voltage so I just could cut the RCA cables going in to the stereo and replace with a 3.5 mm cable instead? then I dont have to have a RCA to 3.5mm hanging from the back dvd to my phone in the front or maybe i should get a new headunit with bluetooth support...

Is there some way you could buy a headunit that could use your phones google map instead of own navigation?..

Anyway back to first question what is the red cable upper right corner suppose to do?



http://postimage.org/image/s9e4myu8h/

Big Thanks.