1997 mustang gt power drain

I have a 1997 ford mustang GT with 140k and 4.6L SOHC. I am running a Optium(sp) battery and the only aftermarket electric gear I am running is a radio, undercarriage lights, and fog lamps. all of these have been checked and draw no power when the key is out of the ignt. well here is my problem. The battey slowly discharges over a 12 hours period. I have checd everything from the starter motor, alternator and all the fuses. but I came across this just today. The key was out of the car and upon check the starter with a multi meter it shows 12.5 volts. I am understanding that this should read 0 volts. so my thoughts are to what switch is open feeding the starter with power. I have come up with 2 things but my be wrong so correct me if I am.
1. Starter relay
2. Ign switch
I am leaning more twards 1 but need some help in finding the relay. the Haynes book I have doesn't show this. any help in locating this item, cost, how to test or just woundering if I my be wrong would be greatly accepted.

thanks again