this should be involved with alt/ecu/bat thing. when the ecu detects bat is charged, demand is low, it turns off the alt. when demand comes back (a/c clutch) the lights dim. the ecu detects this, and turns the alt on, so the dim stops.
adding the grounds prly wont help
choices
1) add a capacitor to power supply, like 1 farad (big ****er), to power erytang while the alt comes up
B] dumb system: cut alt wire to ecu, splice to whatever its 'posed to
3} deal with it. my cars sill mostly fine, all unrelated after about 100k in almost 8 years.
adding the grounds prly wont help

choices
1) add a capacitor to power supply, like 1 farad (big ****er), to power erytang while the alt comes up
B] dumb system: cut alt wire to ecu, splice to whatever its 'posed to
3} deal with it. my cars sill mostly fine, all unrelated after about 100k in almost 8 years.