Sv 650 no power at rear coil
 

Hi all please help bike started missi g so changed plugs and coils still missing so narrowed it down to rear coil front coil has 12 v rear has no earth signal but if i earth it on frame i get 12 v
So traced earth wire back to rear of bike its a white wire goes into a box under rear light
Wire is fine so what it this box anybody have any ideas for me im lost now
2001 sv curvey