So I was thinking about putting a vw 2.0t into my 1999 eclipse gs. Would I have to get a standalone ecu or can I wire in the engines ecu in place of the eclipses oem. I'd be able to figure everything else out on my own but electrical is my downfall
There are a few different ways to go with what you are trying to do.
Piggyback the original ECU and use a VW ECU to run the engine. If you want the rest of the vehicle to work, you'll need the original ECU to display activity to the stock dash.
Replace the original ECU with the VW ECU, then modify the gauges to accept the input of the VW ECU. This may entail complete change out of the all gauges. This is your standalone solution.
Replace the original ECU with an aftermarket ECU as well as gauges.
In all three situations you'll need either the VW ECU or an aftermarket one to do the deed. Someone may have already done the swap you're talking about. Do your research. If it has been done and someone has recorded what needs to be done, it'll give you a leg up. Don't reinvent the wheel if you don't need to. If you go this route, you'd want to look at the Eclipse forums as well as YouTube for the information. Particulars would be well beyond the scope of this site.
As an example, my son blew up the engine in his Impreza. It was a 2.2L 4-cyl boxer style (EJ22). He had heard you could replace it with the 3.3L 6-cyl engine out of a Subaru SVX (EG33). I looked around on the internet and found someone who had done it and followed his example. It gave me what I needed to retrofit and what to buy to make it happen. Ultimately it was a fun project and there was very little guessing to make it work.