Hello & Welcome to Just Answer,
Sorry I didn't see your question earlier and to hear of the issue with your device not powering up.
May I ask:
1) The AGE of the device.
2) HOW LONG was the device allowed to sit in the sun?
3) Has the vehicle power adapter fuse been checked, along with the contacts at the mount?
4) Have you tried to charge the device via a powered on computer, and the USB cable - for at least a couple of hours?
If needed, Batteries plus MAY have this battery, or perhaps they could order one for you, and they may even be able to replace it for you.
But if not, you can get a battery replacement kit (battery and tools) from the following link. >> click here <<
You should even find a video right there on that page that shows you how the job is done.
Please let me know what you find or if you have any more questions or details about this device.
Thanks, ***** *****