To me this says that the alternator case is NOT grounded to the level it should be. Why? Because it should have instantly blown the fuse. Suggest going back the "how to perform a voltage drop test". I don't think the ground between the alternator, motor and battery ground is STRONG.

Another test is to take a battery jumper cable and go from battery negative to the alternator's case.

The Crown Vic's mainly use a Ford 3G alternator. Where as the Mustang uses the 6G version. See the "howto"for some information about the difference. However it is possible to use a Crown Vic alternator in a Mustang. The interchange is not across all model years. As some Lincoln models use an extra terminal that "tells" the alternator to turn on. However, I not totally certain this is your problem as the Mustang Alternator should put out IF it has power(s) and ground.

NOTE: the interchange does NOT apply when talking about PCM controlled alternators.

Note2: You have confirmed there is "always on" power at the alternator's main B+ terminal right? If not the main fusible link is blown.

Howto perform charging system voltage drop test
https://www.stangnet.com/mustang-for...-drop-test.56/

It DID blow the fuse in the wire as well. Jumping the neg terminal to the alternator case had no effect. All my grounds are basically brand new, since I just built all new battery/ground wires in March or April.

I did voltage drop tests, but the stupid notepad app I put the numbers in didn't save it so I can't give you the exact numbers. Engine off was around .6 B- to alt case, and 1.4 B+ to alt B+. Engine running was something like 4 and 6. The battery was pretty low, so I hooked jumpers up to my 07 so the system was getting proper voltage, and then it was about 29 and 31. This was all on the 200m scale.

The Crown Vics look to be similarly wired up to 02. In 03 they were rewired to go straight to the PCM instead of the charge light and then junction box. See this;
Ford Crown Victoria Alternator Wiring Diagrams

The part number on the case is for an 03-05 Crown Vic (according to Oriellys). If I'm reading those diagrams right, that's the start of the PCM controlled alternators.

The fusible links are fine. One of the first things I checked, and just rechecked.

Update: It did end up being the incorrect alternator. I took it back just before the 30 day return period expired, and ordered a new one for an 03 Mach, from Rockauto. Put it in today and it's running and charging perfectly fine, holding ~14.7v while running.

Will probably have to drive it a few days to get the battery charged back up now. Good excuse to hear the 4V again. The 3V leaves alot to be desired.