Looking for a solution, possible ground fault, RPM does not work

[70and80]

First time user, i have a 1993 FJ80 and the RPM gauge does not work continuously, but when the window is up and you keep your hand on the "up" position on the window, the RPM gauge works. I am going to assume some type of ground fault...i don't know where or how to start to find this fault, any suggestions?
Any information is greatly appreciated.
Thanks, 70and80

 

[jonheld]

The tachometer is fed by pulses from the igniter. It gets +12 from the GAUGE fuse and is grounded within the cluster.