How can I get my iphone 6 stay connected to the bluetooth in my car?

That is a question. However, to be able to determine a problem, what happens when you have it connected to the car? Are you doing something when it disconnects? What are you doing when it disconnects? What troubleshooting have you tried? What model and year car is it? Have you talked to the vehicle manufacturer about the issue and what did they say? The failure to maintain a Bluetooth connection is not always a telephone's fault, it can involve the vehicle.