We bought a new wire harness for our 71 charger that was made for stock ecu.. went to put it on the car. Few days ago and noticed that it looks to be for a 2 prong ballast.
I have it plugged into the ecu.. But I thought had to use a 4 prong ballast. I'm not %sure as I bought it few years ago. But think I bought it at a Evans booth at the nays
....does it need 4 prong or 2?

That depends on the ECU choice. A 4 pin ECU only requires a 2 terminal (single) ballast but with your 2 terminal harness you can use the 4 terminal ballast if that is what you have but it would look unsightly (plug the 2 female terminals into the .5 ohm side) & you might ohm both sides seperately & depending on your coil choice you might parallel the 2 resistors of a 4 terminal ballast to reduce the final ohm value (my last parallel one was 1.1 ohms). An OE 5 pin ECU requires a 4 terminal ballast/harness. A parts house 4 pin ECU is the same as an OE 4 pinner (as far as connections go/performance my vary) & only needs a 2 terminal ballast & a parts house 5 pin ECU with a dummy (no continuity to anything) 5th pin also only need a 2 terminal ballast but can use a 4 terminal ballast (same as prior, use the .5 ohm side or parallel both sides). Short version: what coil/ECU are you working with? EDIT & ballast?

The box on it now is not a factory ecu...and may buy a new one. (Or paint this one? Car is fresh painted and want all to look new/fresh

I'm sure new OE appearing repro ECU's are available.

I don't have to look like date code or anything. Just nice lol

The one on it now is orange and all scratched to crap. Was the only one I could find In my junk wiring harness I had laying around. Don't remember if it had 4 pins or 5

the MP orange box would have 4 pins which would work perfect with your "2 terminal" harness/2 terminal ballast. If you get another 4 pinner (new I am assuming) of the type/color you like then you're all set.

you still can use the 4 prongs ballast... if you see the ballast will notice it got some slots. 4 prongs ballast plugs got a key/stud to be sure will be installed allways on same indexing, to be sure the high resistance and low resistance paths will be propperly conected to source whichever have to source.

On a front view, if you fit the ballast with notches/slots toward to the driver side, use the top prongs to the new harness.

Other wise, just get a single ballast ( 2 prongs ).

Thanks guys. Great help