first time problem sub is pulsing

hi guys i have a problem i have not had before which is the sub keeps pulsing.. the details the car is a bmw7 series 2002 with the idrive and nav system. there were no signs of an amp system but i can not be sure at this time. i hooked up to the 2 rear speakers in the shelf behind the seats and used a simple low level converter from autoleads with left, right and earth connections. the earth i took to a chassis ground near the battery which is the same place i grounded the AMP. now with this setup plugged in it pulses. with out the rca plugged in its fine and it also fine with another source plugged in via rca to 3.5m jack. anyinfo regarding this or what the problem is called so i can search up on a cure would be great thanks karlD6-In Car Solutions

after a deeper look into it, it seems to pick up on the electrics when they "wake up" and pulses, have any one had this problem whilst installing intoa 7 series?D6-In Car Solutions

An amplifier "pulsing" is almost always a symptom of a bad ground or input gain set way too high. I'd be very surprised if the 7-series did not have a factory amplifier, by the way.Support the12volt.com

Does the simple line out converter have any wires on it that are not connected? If so, what brand is it, and what color are the wire(s) that is/or not connected?