I just had a stereo install and now for some reason, my speedometer is 8mph off and the tach only reads 100rpm at an idle. Any idea why?

Not sure if that install would have anything to do with it but I thought I would say I had it done.

When my car is off, the speedometer gauge is straight down, basically about 1 inch from the "0mph" mark.

The tach is in the same place. Everything else seems to be ok. I also noticed that the light behind the tach is not working either. Occasionally if I "tap" the cluster it will come on.

Obviously that is a loose bulb or something like that but I would like everyone elses opinion.