The tubes (with o-rings) come through the firewall to the heater core on the left side of the heater box located just to the upper right of your gas pedal. Pull the carpet back and remove the plastic shield (held in by one screw towards the front on your car). You'll then be able to get a good look at those tubes going into your heater core. If it's still not obvious, have the cooling system pressure tested to located the exact spot.

The heater core and/or those tubes (w/o-rings) can be removed w/o removing the complete dash if need be, tricky but I've done it. The bad news is that if it's been leaking alot of coolant for some time, there will be a sea of coolant under the insulation mats below your carpeting and being that it won't just evaporate, the solution is to remove the interior parts (seats, carpet), the insulation and possibly replace both the insulation and carpeting if it can't be cleaned somehow. I once saw that happen with a new car and it was a nightmare.