2003 Saturn Vue Need advice on how to trouble shoot electri

I just did an engine swap on a 2003 VUE 2.2L AWD automatic.

I have everything "all hooked up" - but it blows the ecm/tcm 10 amp fuse when I apply power by hooking up the battery.

After some head scratching, I eliminated all the connections to one. If the engine oil pressure switch (EOP) is left unplugged, the fuse won't pop - but of course, the engine won't start.

I checked continuity along the wire (light brown / black) and when the wiring harness is unplugged, it doesn't show a short to ground.
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SO, I'm stumped. Electrical is not my strong point (can you tell)?

I'd buy a replacement engine wiring harness if I knew that would fix it, but I hate spending money without some assurance of the outcome.

Suggestions?

Thanks, in advance.

Kim