Strange Subwoofer issue

Hi, 2012 xfs 3.0d portfolio with bowers and Wilkins stereo. Recently noticed that the subwoofer hasn't been working and all the bass was from the front speakers. During the recent hot weather the subwoofer started working? The extra bass shaking the car made it obvious it was suddenly working and now I've noticed it doesn't work in a morning when the weather is cold on the way to work but after the car has been sat in the sun all day the subwoofer works perfectly all the way home and if I go anywhere else in the afternoon but then nothing the next morning again ? Guessing a bad contact somewhere. Anyone else ever had this ? I've checked the connector in the boot and reconnected it and checked the 2 wires going to the battery from the sub