I'm thinking you might be refering to the evaporator coils, which are under the dashboard in the same general area as the heater core. If that's where it's leaking and you have been adding dye to your system, it's possible you might see a few drips or dye coming from your A/C drain tube on the firewall. That's how I located a freon leak on an '83 T-Bird I had. If you still can't find the leak you might have to go to a shop that has a sniffer which is able to detect trace amounts of R12 / R134.
Coils? What are you calling the coils? I am assuming the Fox body in your icon? There is the condenser that sits in front of the radiator and the evaporator that is in the heater box. The condenser is easy to get to. The evaporator means you must take out the heater box and cut it open to replace.
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yea, it's the evaporator I'm thinking of. It takes about 1-2 weeks to leak down from a full charge to zippo! We did put dye in it last charge, couple of weeks ago, but I don't see anything under the hood, so I'll look under the dash for the dye. Thanks.
If it is the evaporator, you will not see dye underneath the dash. The only place that you may see it (if enough leaks out) is the A/C drain at the firewall.
Call around and find someone or a shop that uses an electronic leak detector. It would take one minute to find out if it is the evaporator. No, do not ask me how I know.