coolant leak, help

Well, I noticed a slight coolant leak the other day so thought I'd check it out today after work. I just got home from work, went in and ate some dinner then came out to check under the car and found a bunch of coolant. The leak is on the passenger side, I thought it might be from one of the heater core hoses up top cause I had to tighten one of them before, but they are dry. Looking around for any coolant, I see a hole located on the firewall. It appears coolant is leaking from there. First off what is this hole for? I have a feeling its for the heater core! I did that fun job at the most a year and a half ago, dont want to think about doing it again

So, does that def mean one is bad??

If it is the heater core- Ive done the heater core before, but I started with taking apart the dash and stuff. I was just looking at my chiltons for 89-93 mustangs. It says in here that with a/c theres just a few steps. No a/c cars it lists all the steps in taking out the dash..... is it suppose to be easier with a/c and I can get to it without taking so many things apart? or is it still necessary to move the dash out a little, etc?

While im at it- recommend the best heater core? Last one I put in was from 50resto, cant believe it could only last this long. I even try not to use the heater, etc. maybe that was the problem.