Tach sitting below zero

just bought a 2010 Challenger R/T last week. I am in the process of installing my radio from my old car into this one. I pulled the instrument cluster to run the microphone wire behind it. Long story short... I went to put it back together and noticed that the gauges were sitting below all the markings. I got everything reset. But now the tach is sitting below zero rpms even when i turn the key on. I have tried resetting the cluster with the trip button. It doesn't help, i manually rest it back to zero with the key on. I turn the key back off and it stays good for about 5 minutes then it falls back below zero and doesn't come back up when i turn the key back on. Need some help on how to get it to sit right. This is the last thing i have left before i can finish putting the car back together.