Thanks for info from previous post about car going dead sitting. Replaced voltage reg. same thing Un plugged voltage reg. over night battery didnt run down until I hooked voltage reg. back up then it doesnt have enough fire to start. Any more thoughts would be appreciated. Thanks Steve

 

You've got a drain somewhere. You need to get in there with a test light or a multimeter and start checking circuits on the fuse block one by one.

 

+1 Tad knows his stuff.

 

Waffles, are you hitting on me?

 

Starfury is right, you need to invest in a DMM (Digital Multimeter) and check the connections, the easiest way to check for shorts is to move the switch to the diode check, it looks like a horizontal line with a solid triangle and another line intersecting at 90 deg with the first line:
http://ocw.weber.edu/automotive-tech...detestselector

the buzzer will go off if a wire is shorted together, to test put the 2 DMM leads together, the buzzer will go off.

Take one of the leads and put to the positive terminal of the battery and the other to the negative side of the battery, if the DMM beeps you have a short between power and ground, the only way to really fix it is to get wiring diagrams of the mustang online, which are widely available