Why do you change them? Did you get a post cat inneficiency code?
The post cat sensors should connect on the left side of the engine (driver's side) - opposite of the pre-cat sensors (mirrored, but on the other side of the engine).
You will need to have the car jacked up safely on stands, then once the old ones are out, you will need to fish the electrical cable & connector with a string or a wire. You might want to observe the original routing.
Looking for a DIY? Parts? Check this out, it might be your ticket
TMS underdrive pullies - Stewart WP - PSS9 - Beisan Vanos seals - Zimmerman cross-drilled & Akebono Euro - Deka 649 MF - 55w HID headlights - 35w HID foglights - Hualigan double din - ACS (rep) alu pedals - Euro central storage console - Breyton Magic Racing staggered wheels - M5 bumper - M5 steering wheel - Tint
Stable: e39 M54, e53 N62 & Tribby