Depends on the year.
From '90-'93, the tach signal comes from the igniter, specifically the yellow/blue wire at position D.
For '94-'95.5, it comes from both coils, the black/white wire at each in parallel.
For '96-'97, it comes from the ECU, a black/white wire at position 4L.
For '99-'00, it's at the ECU, green/orange at position 2K.
For '01-'05, the green/orange wire is at position 3O.

On the NA, there is a single-wire sensor on the back of the head to feed the dash gauge, and a separate two-wire sensor for the ECU. On the NB, both sensors are combined into a single housing with a three pin connector- two wires for the ECU sensor and one for the dash gauge. (In all years, the dash gauge sensor grounds through the body of the device, whereas the ECU sensor has an isolated ground.)