heater core replacement

OK, I've been reading the forums for a while now with regard to changing out the heater core, now that it's getting cold the time has come to change it out. I have the fog/mist coming out of the vents and the top radiator hose is stone cold even with the temp gauge reading 190* so i think its time to swap it out. Pass side floor is dry but if I turn on the defroster the windshield gets coated with antifreeze film. I was wondering if this job has already been done on my truck? I'm the third owner and it has 140k its a 1997 2500 4x4. The reason I ask is that I already have heater hose running from the engine hard lines directly through the firewall and I wasn't sure if this indicates that someone has already been in there? I thought the heater core hard lines would have come through the firewall and been connected with heater hose under the hood from the core lines to the engine lines?