It is in the fire wall behind the heater box.
There isn't a block heater, unless the previous owner installed one.
Get a manual on your car and do the job correctly.
Normaly it would be expensive to have a mechanic fix it because they normally charge you per 1/2 hour or per hour, but there are some out there that can do the job with out charging you an arm and a leg.
Behind A/C and heater control panel, in center console.
It's part of the heater core assembly so it's behind your glove box
The heater core is located behind the dash inside the heater core box with two ports, an inlet and an outlet port protruding through the firewall into the engine compartment.
you should hire a contractor to inspect your water heater. it's probably very expensive to fix and you can't fix the broken water heater yourself. too bad.
your heater will be cold, or your engine can over heat. your car can take a long time to heat up. Your inside heater will be cool, or your engine temp. will over heat.
You need to remove the dash panel. It will allow you to get access to many connectors and it will be easier to remove the harness.
Trouble code P0135 means:O2 sensor heater circuit malfunction (Bank 1 Sensor 1)
Not exactly sure with the eclipse most vehicles are inline with the heater hose, and on or near the firewall