Alpine 3539 faulty? (DC between RCA shield and supply ground)

Hello everybody,
I'm not here from a long time but maybe you can actually help me.
Let's talk about my alpine 3539 car amplifier, just bought used some days ago.

The problem is that I measure a DC voltage (more or less 12Vdc - no AC present checking with the oscilloscope) between the ground of the main supply and the ground of the audio stage (RCA input shields). I've studied the schematic of the amplifier and the two gruond should be separate (floating ground on the audio stage, so no DC between the two grounds). There are only some capacitors between the two grounds.
The voltage is more or less 12Vdc - no AC present checking with the oscilloscope. Obviously when I install it in my car, I hear loud bump and other noise.

Have you got any idea? What should I check?
the amp is working properly connecting any battery source like mp3 player.
I've already checked some main I.C. but are ok.

Let me know if I can post here the service manual or some abstract.

Many thanks to everybody who can help me.

Michele - Italy
OK, many thanks.
If a moderator would kindly shift my thread on the car audio section I will thank him.
However, I've got some news.
A service center suggested me to check if the secondary winding of the DC-DC transformer is shorted to the primary. The transformer is ok: there are some hundred of kohm between the two windings and I do not have any DC between the RCA shield and the negative supply of the amp when it is connected to the supply but switched off (remote disconnected).
I've re-checked all the main ICs. I've found a defect on the IC801 that is the PWM ic. This is an alpine branded IC with 20 pins. The pins 19 and 20 are the feedback from the output rectifier stage of the secondary of the DC-DC transformer that supplyies the audio section of the amp. On this pins I should find something like 0,6 and 1,7V (I don't remember the exact values). I measure about 18V.
I think that I should find a replacement of this IC but this is a serious problem.