eletrical problems, no power throught car

hey hows it going, have a qustion i need to ask as i am stumped on the problem. i have a 85 mustang gt, the other day the car would just click when turning over, so i put on a new selonoid swithch and that dident help, but now when i put on the key the power comes on but 2 seconds later all power goes away, if i turn the key off then on agian power wil come back on but then go right away, any ideas, thanks

where does that other end of the pigtail on the gound battery go? also the battery is not that old, just a few minuts ago i put the battery charger on the car and now this time i have real loud clicking coming from the selonoid switch (i just put that on this morning, its brand new) but with the battery charger on the car if i turn on my radio it dosent have enought power to play the radio, also the signal lights, when i turn them on it just blinks once and quits, there is just not enough power to run them, and that it with the charger on there. what would be causeing the problem, i mean what would stop the power from entering throught the car? would a bad starter cause this to happen? voltage regulator? i am not sure. thanks