Go to your electronics store and get the mating 3-pin plug.

Then you can unplug from the CD player and plug the car cable direct into your ibus interface

The 3-pin plug is a standard size, I bought one from Jaycar locally for about 40 cents.

What I do however is easier - i use crocodile clips and clip onto the connector behind the amp, no need to disconnect anything, and it works :-)