In answer to your specific question you are allowed to come and go with your vehicle within the six months, providing you don't exceed six months in one year. Your vehicle (which is in your passport) will be 'logged' out of the country with you, when you leave.
This question crops up a lot. I had thought you could only bring your UK car in only twice in total; for six months limit in a one year period. However other members have said that they know people that have been more than twice. I also took the trouble to check as you have and cannot get a definitive answer.
The best I can find is paragraph 11.4 on this link
Buying Property - Residing in Turkey
I quote; " 11.4 Can you bring your car to Turkey?
If you visit Turkey on a tourist visa you can bring your car into Turkey and keep it up to six months in a period of 12 months. if you were to leave Turkey for some reason during your stay in Turkey you have to either take your car out of the Turkish Customs Territory or leave it at the nearest customs depot. If you choose the latter there is customs depot fee applicable."
As you can see it makes no mention of only two visits. I would read it as six months EVERY year, but It may not be. One thing I would mention is that every summer the roads are full of holidaying Turkish/German Gastarbeiter who return temporarily each summer presumably with the same foreign registered car.
Sorry I cannot be more helpful or certain; perhaps in this case it would be best to snail mail the Turkish embassy.