There is a cover plate on the front bank of the six that could possibly be the mount for a block heater. I am not aware that Toyota offers a block heater. Maybe.
The block heater is located near the engine on most cars. This is true also for the 2003 Toyota Rav 4.
Block heaters are not installed by the manufacturer. So, unless a prior owner installed one, you do not have a block heater. If you do, it will have a standard 120 volt AC plug hanging somewhere.
They are installed in holes in the engine block where the frost plugs are normally located
1990 Camry 4 cylinder thermostat is located off the bottom intake hose connected to the engine block.
The 1994 Toyota pickup heater relay location can be found in the fuse block number 3. The fuse block is located above the glove box and can be seen when the glove box is removed.
The Pontiac G6 block heater is located on the right hand side of the engine. The block heater can be located by following the block heater cord.
behind the fuse block
Knock sensor is located in the wall of the engine block facing the firewall. You have lift the car if you want to reach it.
A block heater is not a standard feature, and few manufacturers offer them as an option. Usually, they are installed by the owner, or for the owner at a shop.
The block heater in a Toyota Highlander can be reached by going through the left wheel-well, toward the drive shaft. It is seated in a hole in front of the rear drive shaft, and can be seen clearly from underneath the vehicle.
Cars do not come with a block heater normally, it is something you have to have installed aftermarket