I have never installed them, but
from what I understand they go in the back. The back o2's compare the gas after the cats (effluent) (if you have them) to what went into (feedstream) the cats and tell you if the cats are working or not (engine light or not).
Rear. Those sensors read the efficiency of the cats. When you install an O/R pipe you need to trick the ECU with the MILs into thinking that the rears aren't reading the same as the fronts which would cause a code or 2...
ok thanks. ight came back on. Im thinking its throwing a code so Ill have to get it scanned this weekend. Its kinda BS how the Steeda Mils don't fit perfectly (you need to trim the plastic)