I am at the end of my teether now

We bought an Aux lead from the shop, £30, My husband connected it to the rear of the stereo and tried it out...Dont work!!!

No Aux
We went back to the shop and explained this didnt work, so the guy in the shop made some calls. He says because the stereo is older than the car??!!! The car is 2006, think he said the stereo is 2003...I will need something else...a box unit for £55. Now this is getting expensive!! All I would like is to listen to my music on my iPhone through the speakers in the car without it burning a huge hole in my pocket.
My husband even made a lead with Capacitors and resistors and this STILL didnt work
Help is needed please