fuel pump running with key out?

[Philly]

Okay guys, im at a loss for ideas here.

Last november my car took a **** on me, as it has done a few times in the past. It killed my brand new battery one day, i boosted it, all was well for a day or 2. Then it happened again and wouldn't start. It had done it before the same issue, which rectified itself with little help from me. This time though I could hear the fuel pump going and some humming from under the hood. I swapped a few relays trying to isolate the problem and ultimately ended up seeing smoke coming from the harness box. My frustration hit an all time high and i shut the hood and let it sit for the next 4 months.

Now, I had a hacked up engine harness from the guy who I bought the motor from, so i attributed it all to that...
so this weekend was the first chance I got to do anything about it... i spent all day replacing the wiring harness...


So i got it all back together and hooked up the battery and once again, with no keys in the ignition... the fuel pump still runs by itself and that humming from under the hood is still there.

I started pulling fuses, one by one, to see if i could isolate the problem area. That did nothing.
I have a pile of used relays laying around, so i started by seeing if any of those were the culprit. Nope, relays are good.

Ultimately, I left one of the main relays (3rd one, closest to firewall, under the black cover, that neighbour the DME and Fuelpump relay) unplugged and connected the battery. No noises to be heard with this one out. A quick check in my bentley says this relay controls the oxygen sensor heater or something like that....now that was a problem area before i swapped harnesses but i assumed the new harness would cure this problem but it hasn't.

Ive googled and looked at alot of things pertaining to the same issue on other vehicles, and Oil Pressure sender switch comes up often...? do you think that is likely, or something else?

Does anyone have any ideas for me? Im out of ideas and I dont just want to throw $1k worth of parts at it and end up in the same boat.

Thx
Ryan