I have a 71 T120 and I have always had an issue with leaving the battery hooked up for long periods of downtime. However, now i can not leave the battery in overnight without it draining to the point of not being able to start it. The alternator is fine, I am getting normal amps when in idle. I have the original Zener Diode, which I hear are terrible, could that be where my problem is? I have no idea what could be drawing the current? Any ideas?