Docapi

Just bought a 1996 t100 for my kid. Previous owner replaced the exhaust with a straight pipe. I put in a new cat back exhaust, but there is no cat- so no downstream o2 sensor. I built a simulator, but can't figure out where the wire harness runs inside the cab. Can somebody point to the right spot so I dont have to tear the whole interior apart looking for it?

*edit*

I decided to wire in at the plug at the rear of the transmission.
the plug has 4 wires:

white/red
red/green
brown
red

I know that 2 are for the heater circuit and the other 2 are sensor signal and ground- but which is which?