I just did a swap on my 99 civic ek and I can't find the plug for the secondary o2 sensor... I've heard it's inside the car? But where?

 

It's unlikely that you have an EK Civic. Check the car VIN. Do you have an EJ8, EJ6, etc.?

 

If it's not an EX it ain't there. You'll have to run wires or swap out the cabin harness, can't remember....

 

It's a dx... where would I have to tap into the harness at?

 

That's an EJ6, which had the D16Y7 engine with the dual header/cat system at the front of the engine. The connector is likely absent now if you have an OBD1 engine harness.

 

google search 'ffsquad' for the o2 wiring solution.

 

What year/trim engine harness did you use ?

 

2000 civic SI

 

There's no secondary O2 sensor connector on this^ harness.

 

I'm aware, it's on the cabin harness which i didnt swap. So now im just trying to figure how to hard wire it to the ecu.

 

The O2 sensor has 4 wires --> Two for the O2 sensor, and two for the heater element.

Three wires run directly to the ECU --> Both O2 sensor wires, and the ground wire for the heater element.

One wire (power for the heater element) gets battery voltage via 7.5A dash fuse 15.

O2 sensor pinouts in the ECU connector can be found at the link below or in the 96-00 Civic factory manual.
http://www.phearable.net/information/tech-area.html