I have something that looks like it should work that came with (it even says Aux In), but the head unit refuses to switch on the RCA leads when I switch to AUX, like it's waiting for a $100 signal. Rrrr.

If it has another plug [bus] beside the RCAs, it's quite likely that a CD changer or something similar needs to be present on the bus for the RCAs to be enabled.

Yep, a litte googling and I found the gnunilink project on sourceforge - looks like the HU definately wants some sort of response from something on the bus...

/Michael