Stereo plug R4 - what's it for?

[Stuey2uk]

Fitting a stereo in to a empty console
- every thing AOK - all working
BUT I've got a spare 6pin white plug - R4 with only one blk/wht wire - anybody know what it does ?

[shnazzle]

Fitting a stereo in to a empty console
- every thing AOK - all working
BUT I've got a spare 6pin white plug - R4 with only one blk/wht wire - anybody know what it does ?

Puzzled. R4 looks like illumination in dash area. Is everything there lighting up OK? 

HVAC controls, buttons, etc?
Only thing is, wire should be green.
Black/white seems to be ground wire

[paulj]

Looking at a photo of a Toyota to ISO harness, it appears the rear speakers are fed through that second plug. If so, would make sense why nothing but ground is present for us.