[fahyim]

Changed the battery but it wouldn't hold a charge. There is a third smaller wire with red plastic that I think needs to be hooked up near the larger positive cable. I just don't remember exactly where. Anybody know where this connects to?

 

[azuriah]

um... i dont know much about mustangs srry

 

[harailt]

Should go to the + battery post. Most likley this is the alt cable.

 

[bagwunagijik]

Not holding a charge could be a short and some thing left on.
Clean the battery with a pasty bakeing soda and dry to make sure all the acid is gone.
Now wile removing the battery cable slowly look for a spark.
If there is even a small one. eather some thing is on or shorted.