Starter motor not working, new replacement installed

[Dattebayo]

The starter on my Sentra stopped working.

I changed the starter motor again, but this time it didn't run at all. I can hear the starter relay click next to the fuse box inside the cabin, I can hear the solenoid click next to the starter motor, but nothing happens after that. I checked the battery voltage under no load, it's at 12.6 volts, which I've been told is fine, and nothing happens when I jump the battery either.

Basically, click, click, nothing. I reiterate that the starter is brand new.

[Dattebayo]

I think I should mention that the starter stopped working after I de-greased the block. I didn't get anywhere near the starter or it's wiring, tho. The closest I got was the fuel rail and the intake runners which cover it completely, no drip to down there. Also, all the fuses are okay.

[Dattebayo]

It was a bad engine ground.

I got a "switch to starter" cable 4 AWG and attached the negative side to a spare threaded boss hole on the head and it started right up.