Hi, There's a female plug on this harness that I don't have anything to plug into it. Does anyone know what's this for?

 

I should mention it's for a 1965 Inline 6.

 

The 65, and only the 65, had an external capacitor for noise suppression on radio-equipped cars. That plug is for that. 66-up all alternators and one mounted internally. You can see it here at the top of the photo:

 

So is that something I should buy and install? I already updated my whole sound system with new radio and speakers. Does it matter if I don't have that part?

 

where can i get one of these from?

 

If you are not getting static through your radio don't worry about it.