I use a face washer with plain water in the bath - I'm lucky DD loves having behind her ears cleaned...
Just make sure that you dry behind them really well. What you're describing sounds like what DD gets when we don't dry behind her ears properly. It goes all dry and smelly. We make sure we clean REALLY well next time (getting rid of the crusties), drying well, and then applying sorbolene several times a day until it clears up (usually about a week). The other spot that can be prone to it is the fold in the neck.

HTH...

MG