I have an aftermarket Blaupunkt MP74 Los Angeles stereo.
It has an Xcarlink adaptor.
My 4s worked flawlessly.
I installed an Apple lightning to 30 pin adaptor and the 5s said - "Incompatible device attached some functions may not work."
The 5s didn't talk to the adaptor .
I tried the 5s with the official 30 pin adaptor on a total of 3 30 pin docks or connectors and it worked on none of them.
The adaptor went back to Argos and the returns girl said that pat of the adapters they have sold have been returned.
They just don't work properly.
The 5s output in digital, the 30 pin output is analogue. Without a proper digital-analogue convertor(currently not available) it just won't work.
General advice is to wait until an aftermarket firm produces a lightning native car kit.