Freightliner starts by itself...

[Chamberpains]

I've seen this problem many times. Check the starter solenoid on the passenger side firewall first. Replace it. They're cheap and easy enough to do. Then before you even say job done to test it, go down to the starter and make sure the wires aren't touching each other or another post on the starter solenoid itself. Do yourselves the favor and just get dirty and pull the wiring off the starter solenoid to inspect them. You shouldn't have to pull the starter to do this. Just make sure you make note of what wire came off which post. Guys misinstall and over turn the nuts on the posts which cause them to rotate onto other wires and posts.

Lastly if your problem isn't cured then chase down the button switch. Its rare to have the swotch fail to cranking. Usually it fails to no crank.