After having had another manufacturers brand of head units i have returned to Sony for headunits and got myself a MEX-BT4000U for the bluetooth and hands free option to fit into my BMW "57" plate 3 series E90.
Having connected it correctly using various harnesses for audio, steering wheel stalk and reverse beep thingy and used the extra long yellow 12V constant cable from the unit to the fuse box for a constant 12V connection, I have stumbled upon another very odd phenomenon.
After leaving the car overnight or even a couple of hours when i go back to it and try and use the headunit it turns on very briefly (say about 3 seconds) enough time for it to show just "TUNER" before beeping once and turning off, if i try and turn it on, it will only turn on briefly before turning off again.
I have to wait about a minute or two before trying again and it will then work normally, why does it do this??