ECU harness install..hard?

Its not really that hard, just kind of time consuming. I did it when I did my 4cyl swap, had to take the old harness out and put in a new one to fit the 5.0. I think the most difficult part was getting the grommet out of the firewall on the passenger side that connects to the computer.

Otherwise when putting the new one in, most of the plugs will fall into place and the good news is that mostly of all the connectors are different so its hard to mix them up.

It would be a good time to hide the wires while your doing this because everything would be out...

And take pictures before you do so you have a visual reference of things just in case.

I did mine last year. It really isnt that hard, just time consuming. Took me about 2 hours to get it out. It is basically plug and play. Just make sure that there are no connectors left over when you are done. Worst part of the whole job was getting the computer connnector in/out of the firewall.
FWIW- Make sure that you get a harness from the same year car as you have. It will save you alot of trouble as not all the years interchange. They are tough to find, but make sure you get a good harness with no cuts, splices or broken connectors. The first harness I bought was out of an 89. I had leftover connectors and the car wouldn't run. Ripped it out and got one from a 90. Put it in. No leftover connectors and car started right up.