A Ford Mustang does not have a heater coil. You must be thinking of your home HVAC.
Only if there is a problem with the coil pack(s).
How are the coils numbered
On the '93-'97 GT's the ignition coil is part of the distributor. If your coil goes bad you need to change the distributor.
Change the coil of the cyl number 2
C.O.P ( coil on plug ) they are under there. Just remove the coil.
Every time it becomes defective.
pull the coil packs off and get a long socket, or a long extension
First thing is to make sure you have fire (spark) at the spark plugs. If not change the ignition coil.
It's rare a coil pack will go out but it does happen occasionally.
I had a similar problem. It was a coil pack. They are about 80 bucks. Fairly easy to fix. If you know how to change a Spark Plug, you should be able to change a coil pack.
What year, which V8?