Im helping restore a 70 chevelle for a friend and we are trying to get the tach to work. Its a new oe tach. We couldnt find any wires on the car or in the books that were for the tach. When we were looking at a catalog for wiring harness it said that the instrument panel wiring harnes didnt come with tach wire. ??? Anybody run into this. THANKS

The tach wire on all the '70 models i've seen is a separate brown colored wire that hooks to the (-) terminal of the coil ( tach terminal on HEI disributors). It enters the car through a 3/4 inch hole and grommet on the firewall near the wiper motor. Does your new tach at least come with insrtuctions where to hook this wire to the cluster?

Jim

Instrument panel wiring for SS with tach and gauges does include the dark brown wire for the tach. It comes from pin 9 of the gauge cluster connector and like Jim says it goes through a grommet on the firewall to the points side of the coil.
You say OE so I assume you mean original factory tach into a tach and gauges cluster. If you are converting an idiot light cluster to tach and gauges you will have to add 2 wires for the amp gauge and one for the tach and replace the circuit board.
Go here for conversion info: http://www.azendrud.com/gauges/