W124 oil pressure sender questions (yeah, I did the search)

[G-Benz]

While replacing some other items on my W124, I happened upon a severely deteriorated ancillary chassis harness (cracked insulation, etc.) It comprised of three wires, connected via a three-prong connection behind the engine firewall on the driver's side. I determined one of these goes to the oil pressure sender, as I have noticed intermittent operation of the oil pressure gauge lately.

The wires routed just adjacent to the oil filter, and from what I could tell, connected to 1) starter solenoid ground, 2) some sensor on the oil pan, and 3) I would guess the oil pressure sender.

Dealer was NO help on a replacement (said I needed to purchase the $600 kit-and-kaboodle harness ). I have assorted reels of automotive wiring, so I decided to fabricate my own.

Now to the problem. Oil gauge pegs to the highest reading when turning ignition ON (and remains there whether engine is running or not). Goes back to zero when key is OFF.

I don't believe I screwed up my homemade harness (only three wires). At least I know the starter wire is correct. Am I correct about the connection on the oil pan being the oil level sender? And the mysterious one (couldn't tell from my location underneath the car) that that would be the oil pressure sender? Is it supposed to be located near the oil filter?