The best replacement for aux is rca.

The best alternatives to aux are rated as following:
★★★★★  rca
★★★★  optical
★★★★  bluetooth
★★★★  bus
★★★  au
★★★  bluetooth sound quality
★★★  phono
★★  line in
★★  fm transmitter
★  usb sound quality
★  cassette adapter
★  em heat
  usb better
  usb quality

Your alternative for aux is not listed? Suggest!
Speaking about aux, one probably shouldn't forget about cd changer.

Other search terms related to aux and their ratings are:
★★★★★ cd changer
★★★★ mic
★★ cd
★ ipad to stream music in car
You have in mind a companion for aux and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of aux in all years. Click on graph for more.
aux
rca
optical