Sounds like the Alt is churning out all it can to supply the mystery load....
A very good test would be to actually measure the current output of the Alt. in the car at different speeds to determine if the Alt is having to put out more than is normally required with normal system draw...

Eating Alternators is an indication of constantly running near the max output...which will certainly shorten service life....and could be the cause of the lamp as the alt struggles to keep up..........That is why the alt. current output test is so important....

Sounds like you are on the right track...get rid of the extra wiring and reconfigure as close to normal as you can....I think you are close to finding the mystery load on the system...

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1978 Yellow 300D (The Mustard Toad)
1980 Blue 240D (The Iron Toad)
1989 Grey Mitsu.4WD Mighty Max Pickup (Needs a Diesel transplant bad)

(Open the pod bay doors HAL)