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The AAA tech was correct.
Did you fit a new alternator, or 2nd hand?
Did you disconnect the battery prior to replacing the alternator?
That's unusual you had a spark with the negative disconnected.
This only occured when fitting the new one?
How big was the spark?
Have you checked all the fuses in the engine bay?
Ok, makes sense.
Have seen it before many times.
When you are playing with high current power, even more critical that the leads are well isolated.
Now we need the check all the fuses, one or more could be blown, generally the alternator fuse.
How did you go with your problem?
I hope my advice has been of assistance to yo.