So I was changing the O2 sensors in the car and when I went to pull the unit out of its 4-wire receiver, the wires came out the OTHER end (the end closest to the wire harness. So now I'm kind of stuck. How do I get those four wires back into their harness? Thanks!

 

you should be able to push the wires back into the plug. just make sure you put them in the right hole

 

There's a piece that clips on each side of the plug. Remove that piece and the wires should go back in the plug.

 

clip on each side of what plug? the thing they are separated from is a solid piece. Also, how do I know which spaces the wires go in?

 

look at the plug on the other side... there should be at least 2 wires that r the same color on both sides. and maybe the wire colors on the sensor r the same too. after that, then push the wires back into the plug. and maybe put some kind of glue on the back side of the connector to hold the wires in place.