Since this rash has been appearing for the past so many years and is non itchy or painful , hence I suggest you to get it evaluated from a dermatologist and get biopsy skin done. Chafing, eczema and psorioasis need to be ruled out.
Psoriasis is a long-term (chronic) skin problem that causes skin cells to grow too quickly, resulting in thick, white, silvery, or red patches of skin.It is thought that psoriasis occurs when the immune system overreacts, causing inflammation and flaking of skin.
My sincere advice is to get a biopsy skin done to confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.