Where is the intake plenum on a Toyota engine?
The intake plenum on a V-6 or V-8 is bolted to the top of the intake manifold. It has the throttle body bolted to it. The throttle body houses the throttle plate and is located at the end of the tube that starts at the air filter housing. Sometimes the intake manifold on a 4 cylinder is referred to as a plenum. It works the same way and is found in the same manner. Look for the air filter housing, then follow the hose past the throttle body to the intake or plenum.
You have to remove the throttle body/air intake plenum to get to the rear plugs. I believe the plenum is supported with two or three stays which are hard to see. You will need a new gasket for the plenum/intake manifold interface.One post on Toyota Nation Forum says that it can be done without removing the plenum. I think he is skilled at beating the clock at a repair shop.