Hello everyone!
My first post so please don't be too harsh
I have Kenwood KDC-BT838U headunit
Kenwood - KDC-BT838U

Works very well with my 4 car speakers but I have also installed an amplified SW in the trunk. Nothing fancy but works. Connected by unshielded RCA cable to the preamp outputs on the back of the HU.

The problem is that when I start the engine there's unbearable low-freq buzz in the SW (not in the other speakers that don't use that preamp). It doesn't change with music volume and is present in all preamp outputs (also for front and rear speakers). Disappears when I turn off the SW output in my HU altogether.

It is NOT there when I connect my iPod to these same RCA plugs with the engine running, thus excluding SW's powerline and the (unshielded) RCA cables.

Has anybody heard of HU giving such interference on preamp outputs but not main amp outputs depending on the engine running?

ANY TIPS APPRECIATED since I cannot use the SW when I'm driving...