We’ve all heard “make sure you wash behind your ears!” from our grandmothers and such.
And it’s good advice - I’ve noticed that if I neglect the wash my ears, the plastic on the end of my glasses tends to generate an odor.
So what makes the area behind the ears so bacteria friendly?
It’s dark and moist back there. Nothing makes a bacterium happier.
However, you haven’t proven your case that all the badness is caused by bacteria. Sure growing bacteria can smell, but so can other stuff.
For instance, seborrheic dermatitis is commonly found behind the ears. Symptoms include redness, itchyness, oozing, flaking, and odor. Some might mistake this for a bacterial infection but it’s not. It’s thought to be a reaction to overgrowth of a skin fungus and is commonly treated with topical ketoconozole (anti-fungal), selenium shampoo (selsan blue), coal tar (reddish neutragena shampoo), or steroids.
Sounds like a dermatologist speaking! 
Damn you PosterChild and your wiseguy comments.