As someone that has the pre-LCI top-DSP option (the pre or post LCI makes a difference with wiring), regular AUX should work which plugs into the rear of the headunit. Never had to code anything, it already enables the AUX option on the HU.
If you want bluetooth as well and you have a pre LCI top-DSP system, then you'll need the famous Blue music module (non headunit version, so 3+6 CD player version) along with DSP converter module, since the Amplifier only communicates via Coax. LCI models went full digital to the AMP, hence the 3+6 connected working as a plug and play.
double check your amp if it has a micro-coax connection
Bluetooth has a far more superior sound quality in my opinion. Aux picks up too much static for this car.
This system i just explained gets wired in the trunk, via the CD changer cables.