Have you checked your drain to make sure it is not plugged?
its the drain line for water to drain
Condensate forms on the AC coil as the AC runs for a while. AC units are supposed to have a condensate tube that's designed to drain the water. If the tube is plugged or otherwise damaged the condensate will back up and get into the furnace. To avoid damage you should make sure the condensate is draining properly.
Has the condensate pan cracked causing the leak?
If you mean a vacuum switch then yes. Its fairly common for condensate to accumulate in a vacuum switch.
You obviously can't ask a question properly and you have a leak in your plumbing.
It is located on the bottom of the firewall on the passenger's side.
A condensate is a liquid that is formed from the process of condensation. An example of a condensate is the water that forms on the outside of a glass filled with water and ice on a very hot day.
Water builds up in the pan the A coil is sitting in, overflows and goes inside the furnace. May short out electrical parts, could short out motor on fan.
Head to the roof, and check your vent line cover. It's basically a tiny roof for your furnace vent. You likely have a leak allowing water to fall down to the furnace. I would recommend being proactive about this: even though you have a drain leg, it can still allow moisture to get into your heat exchanger, which will mean a new furnace in a few years. Not to be an alarmist, but I'd take care of it soon.
DUST!!!!!
Kinetic energy
A furnace does NOT have water... A boiler has water .. NICE trick question