2002 Mustang Stock System Acting Up

[bbal100]

A few days after replacing my factory head unit with a kenwood unit, a weird buzzing sound started coming out from the back of the car. It stops when you disconnect the RCAs from the deck so I assume it's the stock sub. The whole system is stock besides the head unit. Weird part is that the sound goes away when you pull up the handbrake or turn on the headlights. Though when the engine is on, the sound doesn't go away completely when you engage handbrake/turn on headlights. I've taken it to futureshop and the guy tried installing a sound filter but nothing helped any help is greatly appreciated.

Here's a video of the sound coming on/off:

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