I already posted this in General Elcetronics Chat, but figured it might me more appropriate here:

OK, heres the deal. My friend has a Neuros MP3 player. He wants to use it in his car, which is a 2003 Honda Civic. He got an adapter which gives him RCA inputs that plugs into the cd changer socket on the back of his stereo.

The adpater also has a ground wire that gets attached to the chassis. The problem is that sometimes he will be running off the Neuros battery power, and sometimes he will be getting power from the cig lighter socket. When he is running off the cig lighter he gets ground loop interference.

He rigged up a switch that disconnects the ground to the adapter when he is running off the cig lighter. But having to flip the switch is kind of a pain.

My question is: Is it possible to make some sort fo circuit that will detect the load of the Neuros on the cig lighter, and trigger a relay to disconnect the ground?

He is also thinking of building a box that contains an auxiliary cig lighter and the RCA's, so he can also power his phone from the main cig lighter. This circuit could also be built into this box. That would make it so he wouldn't have to alter the car, which is a bonus for him.

Any ideas?