big tach wiring problems

in the fuse box there are three extra spots to run extra accessories or what not. when the tach is wired into one of these spots the car does not start and it seems like the starter is not getting any power but now i have a switch that turns my fan on and off in one of these spots and the car starts but it hesitates to fire up strong and when the switch is completley unwired from the fuse box the car fires up like a champ somebody, anybody help me thanks

Just Do What I Did. I Put My C2 Tachometer To A Switch And Put A Fuse To It. I Think Its Easy And U Avoid Lots Of Headaches.

i wired everything off my radio. constant 12V, ignition 12V, lighting i got from the dimmer switch, and of course tach signal from the guage cluster

I wired it from the radio too. one question how did u know what wire was the rpm wire from the gauge cluster. cuz i taped it it to the wire from the distributor and i hate the way i did it??

It's the all Blue wire behind the cluster.

thank u very much.