Amp ok until motor is started

Amp ok until motor is started

I installed a subwoofer amp, at the moment I am using the high level input
on the amp. That means I'm running wires from left and right speakers to the
amp input connector. So I have the rear speakers in parallel with the sub
amp inputs.
All works fine when the ignition is turned on, but when the motor is
started one of two things happens.
1) The volume goes way down on the sub woofer.
When #1 happens the rear speaker volume is also very low.
2) or the subwoofer starts a low frequency rumble.
Once the car is started I can disconnect the subwoofer input then plug it
back in and all works well again.
What do you think could be the problem?
Thanks, Mike