bad battery drain

Ok yall, I have been having a problem with my car for a while now, The battery tends to drain completely over night I'm not sure why its doing it but it never used to do it until I dropped in the new engine. the only alteration I made to electrical is a stereo and gauges, as well as a tach. The only aspect that runs direct to the battery is the volt meter its always on, yet I don't see how that is going to drain my battery from 12 volts to completely dead over night and I mean COMPLETELY dead.

Any idea's on what it could be? maybe it actually is my volt meter???