99 Freightliner Classic will not start when I push the button

Look at the solenoid on the fire wall. Determine which small pole goes to ground ( I believe it’ll have a wire grounded to the cab somewhere near it) and which goes to your button and which big pole has power and which feeds the starter. Turn the key on and jump the power pole to the button see if it starts

If not started, grab a pair of pliers that’ll reach both large poles and jump those. If it starts replace the solenoid,

If it doesn’t jump the large pole of the starter to the small pole of the starter. If it turns over normal replace the firewall solenoid or the wires to it. If it cranks slow, you it maybe your start dragging, or your batter cables are burn up. Get some jumper cables and run from your battery to the starter and try it again. If it turns over better replace your batter cables. If the same try hooking the ground side up. If it’s better replace your ground cable. If not, have your starter tested. It may just need a set of bearings.

I got to the point before we sold all of the classics, that if we had trouble like this I would first replace the firewall solenoid, re-crimp and solder both small wires and replace both 8or10 gauge cables between the firewall and starter (everything soldered). If it still had problems (some times a few months later ) I would look into the starter or main power ground cables.