Unpublished specifications are a known drawback of those marvelous sealed boxes. Not mod-friendly, especially when current drive specs are involved. A solid state relay can sometimes be helpful when attempting to add to a basic on/off circuit, replacing the original direct connection. Beware of inductive kick if attempting to use regular relays.
When altering a circuit that drives bulbs, compute the load that the stock bulbs represent, and never exceed that. And cross your fingers.
We have seen reports here of people blowing the drive transistors in the boxes by exceeding the original load.