there is a post (I see this now HFL Blutooth Fix - Relay - No Soldering!) where you connect a relay to the BT unit to prevent it from trying to drain power when the ignition is off - surprised that isn't the default. Why would one need BT to look for signal when car if off is beyond me... that's why the unit gets hot and drains the battery when it goes bad.
The problem is in the electronics. If you see the schematic, the module is always powered up and just controlled by the accessory voltage when the car is in use. But, the module receives power 24x7. The idea is to interrupt the main 12v power when the car is not operating. There is no adverse effects in the memory of the module.
What fails? There is a timer that keeps the module ON and operational 15 mins after the car is turnes OFF for you to keep a telephone call active. After that time, the module goes stanby but it is always powered. When the timer electronics fails, the module keeps full power 24x7 because it does not go standby.
Again, the best way to extend the life is either removing the permanent 12v signal by inserting a relay or using an external timer to cut the 12v after 15 mins and keep the functionality as designed. I opted for the relay and cut the power when I switch the car OFF. If I am on a call when I arrive home, then I keep the car in ACC and turn OFF when the call is over.