gauges

Got some aftermarket gauges for my brother in law and he was installing them and now the RPM gauge doesn't work oil batt, temp, gas work but not RPM. What could have caused this and how can he fix it?
Thanks

I got the same problem, only the fuel guage is what is all screwed up....when I fill my tank, it stays at where it read before the fillup, then if I tap the guage with my hand, it slowly goes up with each tap. I thought it might be my fuel sensor near my tank, but I think something in the dash is screwed. ???