How old is the battery, and what is it current cranking Amps (not just what is on the label but what it measures)?

While it is time for the wire to start needing to be replaced, start with more common items.

 

Starter Relay Diagram

This diagram is the same for both 3.8L and 5.0L MN12..

Rayo..

 

I had the battery tested and it was good. I don't know what else it can be.

 

I would first check the resistance of your wires before you replace them..It's likely they are full of "green rot"

You need to check both wires running from the Starter to the (remote mount) Relay..
As well as the battery cable running to the (remote mount) Relay..

If you are getting a lot of resistance, pull back some of the insulation on the wires to check for "green rot"

It wouldn't hurt to do a voltage drop test on your battery terminals either:

Rayo..

 

A straight resistance test will not reveal corroded wires, you need to do a voltage drop test.

 

Exactly; an ohm or volt meter doesn't draw enough current to see the problem.

Clipping a voltmeter to the starter power connections, and holding it so you can see it when you crank it will tell you a whole lot more.

If it drops a bunch, measure the other end at the battery and megafuse connections; keep getting closer to the battery until it disappears.

If it does it at the battery, charge it overnight and try again.

 

I tried to hook up a new wire for the big starter wire and I couldn't get it to work. Right when I tried hooking it up I put the - back on the battery and the starter starting cranking over and the terminal was sparking. And the starter only had 1 wire hooked up. I took the old starter out and put in the new one that had warranty. I found my volt meter but I don't know what volt setting to set it to. I think my starter died too fast from long crank times, now I will just crank it for 2 secs then crank a second time to start.

 

Long crank times can be eliminated by turning the key on for 2 sec before cranking. Or replacing the fuel pump.

The setting should be V-DC.

Al