[2004 VR, HU is a JVC KW-AVX840, using an eBay DIN to RCA cable for factory amp. Center channel is speaker-level output from HU (LF+ and RF-).]
I have an annoying hum in the audio on my V70. I'm using a JVC HU and the factory amp. When I switch back to the Volvo HU with the same DIN cable, I have no hum. The hum sounds like typical alternator whine with some extra electrical noise thrown in, I can hear the console lights dimming for instance. It's also louder when the car is in Advanced mode, so I can hear suspension chatter too.
I don't think it's the JVC because I had a Kenwood previously and had the same issue. I tried a replacement DIN cable to the amp but that didn't work. The audio coming from my center channel is crystal clear, so I know the JVC is putting out a clean signal. That leaves the eBay cable*. On the cable I'm using all 4 RCAs, for the negative I'm using the constant - that also powers the stereo. For the positive I'm using the JVC's remote amp turn-on wire.
It sounds to me like something isn't grounded correctly and I'm apt to blame the eBay cable, but before I spend money on a replacement, I hoped someone might have some insight.
*one of these http://tinyurl.com/7h9tv2t