for my 79 rx7, my tachometer is not working, when the car is on it justs goes to like 6, then hen there is any power draw, it drops down, but then comes back up when the power draw is turned off. Any one know how to fix this?

 

Also, how hard would it be to install a new tach, like in its own housing?

 

check the coils with a volt meter.

just a recommendation