Don't know how to test the ECM. But.... on my '95 the two plug wires. The 3rd wire goes through the ignition switch to the battery, the 4th wire goes to the ECM. These two wires go to the the field in the alternator. The rate of charge is determined by the current flow through the field. As wire 3 goes to the ignition switch, it is fixed at the voltage of the battery minus any losses in the wires. If there is no current flow in the field AND the field wire is intact the voltage on the 4th wire will be equal to battery voltage. If this voltage is less than "X" a circuit will turn on and allow current to flow through the field. This will allow the alternator to produce current and charge the battery.
To test you must make a circuit. A brake lamp, (combo brake and tail is preferable) socket, clip-clip leads will be sufficient. Connect 1 clip-clip lead to the lamp socket. Now for the good stuff. Take the red meter lead and connect it to the the + battery, black to -. Now go 6" or from the plug and cut wire 4 leaving wire 3 intact. Strip 1/2" from the "alternator" side of the wire and connect either clip lead. Start the car, and check the battery voltage. Take the unconnected socket lead and attach it to a good ground. The lamp should light and the Voltmeter should increase indicating the alternator is charging.
If the lamp does NOT light...check for voltage on wire 3, then check for voltage on wire 4. If there is voltage on 3 and none on 4 the alternator is bad, (open field), if it starts to charge, by elimination, the ECM or associated wiring is bad.
Now, if you decide to go with an external alternator.......Hopefully you gave yourself 6" of wire above.
There are two flavors of regulators, Positive and Negative. One type the alternator field is grounded and you vary the source to determine the charge, the other the Positive is fixed and you vary the ground side. You need the latter. There are usually 3 wires plus a ground. Mount the thing on metal, both for heat dissipation and RF (radio noise) suppression. 1 lead will go to the battery sense wire, another will come from the ignition switch (or ignition activated voltage) If you don't do this, the current flow through the field will drain the battery. and the last goes...............to the 4th wire on the alternator.
If the light bulb works, make a good connection, it will work until you figure out what you are going to do. check the water level, if it is overcharging too much use the tail light instead of the brake light.
Let us know, I'll be off the computer from Tues morn to Wed evening. I'll check in before I leave to see if you're OK.
It is easier to implement than read.