Cat ears dirty or mites??

I’ve had my cat for 3 years and did mites treatment for a while for suspected mites, he’s always had this and has been to the vet when we got him and they didn’t say anything about mites. He’s strictly indoor. any ideas?? or is it just ear wax

Hello- most cats do have one or two ear-mites, which might multiply at times of readily available food source (ie ear wax / sebum etc).  Your question however, seems to be where the 'wax' came from.  Allergy, low humidity, particulate matter in the air, other foreign material and extremes of temperature are common predisposing factors to ear inflammation and wax production. Your vet can help you to decide the way forward.