Hmm. And you're connecting it to the standard Business CD head unit as well right?

This my one: https://www.ebay.ie/itm/Aux-in-Adapt...k/263445006519

It seems to fit perfectly, but then I read some things online about the cable needing to have a "300k resistor" (means nothing to me), or possibly even that the Aux option needs to be coded into the head unit.