Garmin Battery Won't Hold a Charge?

My Garmin StreetPilot c330 GPS battery no longer holds a charge. I use the mount to the dashboard, and the cord has to be plugged into the car and the unit for it to power on. It costs $150 for Garmin to repair it, which is more than I can buy one for!

So, does anyone know a trick to get it to start holding a charge again, or how to replace the battery and what type to buy?

Thanks!