Aftermarket stereo problem

[M7]

My standard VW headunit in my 9n Polo works fine it's connected up to a disc changer too.
I bought a headunit on ebay, 2nd hand, and connected it up with a wiring harness I used to use for my last Sony headunit a couple years ago. The 2nd hand one didn't work. The seller claims it worked fine etc when it was taken out his car. All I got when I connected it up was the cd player ejecting a cd, no lights or functioning.
So I just went and bought another headunit, brand new this time as I couldn't be bothered with the hassle of 2nd hand goods. That didn't work either, again just the cd eject mechanism working when connected up with the harness.
This made me think the harness was faulty, so I went to halfords the took the connector from my new Kenwood, the guy there said I didn't need a harness at all as the kenwood headunit I have connects straight into a VW. He was right, they all plug in but I still have no life what so ever from the new cd player. Not even the ejecting this time when it's plugged straight in.

It confuses me how cd's can be accepted and ejected, so power is getting to it when the old harness is there, but it doesn't seem to turn on. It's really puzzled me now, any help is appreciated.

Cheers

[david burton]

Sounds to me like the car's switched live wire is connecting to your main constant power for the HU, and there's no switched live (remote) feed to the HU. Try getting the constant 12v source and feeding it to both remote and main power on the HU and see if it turns on and does everything.

You can normally eject a disc with the ignition off but nothing else on most HUs, so it stands to reason the HU thinks there is no switched live and your car isn't running (when it is). If this is the case you need to spice the right wires into the harness, or get the constant 12v and put these into the HU as above - the stereo will be able to be on all the time then.

Have you got a multimeter?