Replaced battery and alternator but voltage meter keeps dropping.

You don't have an aftermarket radio or something like that installed do you? My ex fiance didn't wire hers right so it was always draining the battery.

 

Replace the short harness on the alternator. A burnt wire touching another is a fire ready to happen, plus a damaged wire can't allow charging current to flow as well.
Never use any car with a damaged or burnt wire, just going over a bump could short something out.
You really need to find the cause of the damage, before doing anything else.
A short in any circuit may cause PCM problems.
Stay away from alternators on eBay, they are mostly rebuilt in China.

 

#6 may be part of the anti-theft system.
Part of the PATS is in the cluster.
The cluster has a blinking theft light in the gauge cluster, and draws current even when the key is out.
You will always have a small voltage dr,just to keep radio presets, the anti-theft (PATS), and current to keep memory of driving habits store in the memory, and since PATS is anti-theft, removing the fuse may disable the shifter, as you need a signal to shift while stepping on the brake pedal.
It's just like the battery i a PC, the PCM needs power to keep data in memory.
The main CPU in OBDII PCM's is almost as powerful as a home PC or laptop, it can do many more things it is currently assigned to do.
It needs constant current to hold volatile (Erasable) memory.
I don't think that fuse is your problem.
With everything turned off, and the car in a normal parking state, remove the ground terminal from the battery.
Measure voltage between the battery post and terminal. There will always be a voltage drop, but it should be very low.
Then, measure amperage draw at the same two points.
It also should be low.

 

One more easy test.
With engine running, measure voltage from the NEG Battery post to the alternator POS output.
Then from Neg battery to POS battery.
You the alternator should read the same as the battery.
Subtracting battery to alternate4r, should be zero, or no more than .2 volts.
If it is higher, clean the wire from the alternator to battery.
You can also add a length of #8 wire from alternator output to battery, then it w2ilol read zero.
The positive wires fro0m the alternator run all over the charging system and fuse-box supply.
Bad connections add resistance, making alternator to battery resistance higher than the desired zero volts.
Also check ground from engine to body, battery to engine block, and battery to body.

 

See if this helps...
I need a picture of under dash fuse box, 2000 ford mustang - 1994-2004 Ford Mustang

 

Under hood fuses...