For every electrical circuit there has to be a source and a return i.e. (ground, earth, 0V, reference etc...) And since you said that the car is charging properly and that the terminal voltage isn't changing then as you pointed out it is most likely related to the remote starter.
You said that you had tried different sources with no change, but you might have neglected the other half of the equation, the ground. Try grounding the remote starter to a different spot and while you’re in there make sure the ground it's tied to is VERY clean. If you taped into a wire that was already grounded then that's probably your problem. Make a new ground wire from the remote starter and ground it somewhere good.

Hope this helps

Tom