No harness is needed if you have the correct pump.
My Forester looked different than the WRX. I had two quick release hoses (orange clips) 1 electric plug, and one hose clamped hose.
The top orange one is the one that will leak the fuel. I had an old shirt ready so I caught most of it.
Note: The bottom two bolts held on a seperate bracket that holds some hoses you will have to push that back to remove the plate that has the fuel pump.
Here is the inside of the fuel tank. My gas light is on and you can see quite a bit of fuel still. You can also see that other bracket I was talking about. It in in the bottom of the opening and has two bolt holes.
Here is the stock fuel assembly. You unplug the wire harness, loosen the hose clamp and it comes right off. The filter thing at the bottom is held on by a little round clip. Becareful when removing it because it is real small.
Here are the pumps side by side:
The new pump is longer, but if you loosen the hose clamp to the right of the one that held the fuel pump on, you can push the hose on a 1/8"-1/4" further. This will give the clearance needed to slide the slighter longer Walbro pump in place.
The wire harness a direct fit, so it is a very easy install.