Negative voltage to starter signal?

How did you measure it and is the starter working? The Black with white wire should have battery voltage while cranking only. With the pos probe on that wire and negative to ground it will show +12 while starting. Now if you measure it while not cranking the starter it should be an open circuit.

The starter is working yes. I used a multimeter and put the positive probe on the trigger and the negative on battery ground(while the battery is disconnected) and it reads fine. Once I hook the positive terminal back up to the battery both the main power supply and the trigger read +12 volts constantly and when I hook up negative to battery ground that's when the trigger starts to get - voltage. Without crank it reads - .3 something and with crank it reads - 12.

OK so the starter is working and the car runs. Which wire are you calling the trigger? The Black with white wire that goes to the solenoid? If yes then what you are seeing is likely due to a slightly corroded ground causing some stray voltage. Try a voltage drop test on your battery cables. To do that take your meter and connect the negative probe to the negative on the battery, now turn on the lights. Next touch the other meter probe to the engine block at a clean spot. Then so the same to a ground point on the body. In a perfect world you would see no voltage but in the real world you can see if there is a problem with the connections by the voltage you actually read. Normally you might see .5 volts or so under that is great. Repeat the test on the positive power feeds. Now take a simple test light and connect it the way you do when the meter reads -12 and see if it lights. It shouldn't unless your starter has some dirt or brush material causing a current path capable of actual flow. The meter reading you are getting is also why many don't use meters. They can show you false results.

Unfortunately the car doesn't run. I haven't been testing a wire exactly I've attached a picture showing what's showing - 12 volts. I'll definitely try a voltage drop test when I get home and purchase a test light.

OK you put the positive lead on the meter on that, then touch the negative lead to a ground point, you should see 12 volts when you turn the key. with the Black w white wire connected to that post. If you are testing without that wire in place and see -12 then you have a bad connection that is causing voltage drop and that is what the meter sees. FYI, you want the old style "scope on a rope" tester, not the newer LED types, those have their place but the good old bulb types are far mor effective for testing.