2007 electrical demons & wonky taillights

[Nitrogirl65]

Just bought a 2007 and it's apparently got electrical demons. Aside from the central locking & rear windows being out (fuses fine), my taillights flicker when key is in position 2, but cut out completely when I start it up. Bulbs have been replaced with no change. My mechanic has mentioned low battery voltage so I just swapped it out, waited the 20 min and reconnected with key in position 2 in hopes it would reset my front windows that come back down if I use auto-up (again no luck). I originally thought the REM was bad, but I read on another forum that fuel gauge failing is first sign of that, but mine still works perfect. The light flicker is making me think it's a bad ground in the back? Any other thoughts?