Hi Nick. This is something that lots of people have commented on to me. Best bet is to buy a Bluetooth receiver for the old iPod input. Have you go the cable for iPod input? Plug in a receiver like this one:
https://www.amazon.co.uk/Bovee-Wireless-Bluetooth-Interface-Integration/dp/B00B52LLJ6/ref=asc_df_B00B52LLJ6/?tag=googshopuk-21&linkCode=df0&hvadid=310799750591&hvpos=&hvnetw=g&hvrand=2484875893530634565&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9045064&hvtargid=pla-455701326910&psc=1
Much cheaper alternatives online, but BOVEE is a well known make. You then just select ipod on the infotainment screen and pair the receiver with your phone. Hope you are enjoying the XKR.