you dont need to pull the plenum.its just a %@$ to feed all the wires everywhere you want them.but anyways its possible. you could pull it, possibly do the waterline thing while youre at it, but its gonna take more time. just plug in the new harness as you disconnect the old one(I just cut mine out as I went along). that'll give you an idea where everything goes. try to do this with a buddy. much easier to feed the ecu side through the firewall.one guy stuffs and the other waits on the other side to pull it through. good luck!