Any advice for adding an additional rear light?

I'm 4 weeks into commuting with my GSD P00 and I've decided to commute all winter. Problem is, in a few months the sun will be gone by the time I head home and with my kid in the back I want some extra visibility.

Does anyone have any experience adding additional rear lights to the GSD? Ideally I'd like to tap into the power cable going to the current rear light but I'm having trouble finding specs for that. And, even if I could, I'm worried that the additional current draw would overwhelm the lighting power sub-system.