There must be an amp that is still powered in there somewhere. You say the amp is bypassed, is this the DSP amp in the boot or the 4 individual door amps? There is a turn on signal from the head unit that causes the amps to be switched on (there have a permanent supply but stay off until a signal is seen on the turn on line). On an aftermarket install, but still using the amps, that would be connected to the electric aerial line. If you aren't using the amps, then there is no need to power them. They can't be fully bypassed or they wouldn't be connected to the speakers. It all depends on what you have and how it has been wired.