I recently checked with the Turkish Consulate in London what was the extent of ID needed when travelling within the Turkey having entered on a Visitors Visa. This was their response which is the same as the the advice I was given some years ago and which I have carried when in Turkey.
You do not have to have your passport on you while you are travelling within the country. We advise you to have the copies with you. As an ID card you should have at least a kind of ID with a photo like a driving licence
On the occasions when I have been stopped in the past whether in a car or on a dolmus I have had no problems
