Ignition switch problems :(

My ignition switch went out recently...I changed the switch out and it fired up but was running rough so i cut the stang off...Well a few seconds later it tried to crank on its own, so i pulled the positive cable off the battery...While doing so i notice that the small ground wire that bolts to the side of the firewall and goes to the battery negative was hott and started to melt....So i checked the ignition switch connector and it had a crack in it, i changed the connector out, but the small ground wire that i stated earlier is still getting hot for some reason and the car is not getting power through the switch after installing another new one again....Any help would be much appreciated!!!!!! Thanks

The small black ground wire that grounds on the side of the firewall and also grounds to the battery that i stated before....When i connect that ground to the battery the car tries to start on its own with the key out....Could this possibly be a bad solenoid that is stuck????????? Im rackin my brain guys!!!!!!

Thanks for the help man.....I replaced the solenoid and it is acting right now besides the small black wire that i indicated that goes to the battery is getting hot and melting everytime i try to start the stang....What is this black wire for actually....It grounds to the fenderwell and then connects to the negative on the battery??????