Well by rear sear I think you mean the auto sear on an m16 series rifle. That is easy, just remove the hammer and hammer spring by punching out the hammer pin, then pull out. Then use a 1/16 punch to push out the sear pin to remove the auto sear. Do not have to remove hammer, but gives fat fingers more room....
Dang, I thought you were working on a machine gun
(your title says sear instead of seat)
As far as the rear seat, It is awkward, but not impossible, all you got to do is unbolt the brackets from the floor and remove. It is easier to take them out the gate instead of the door. and if you can get help do so, the 60 side is heavy and like I said awkward.