[Zero10]

Hmm, well your voltmeter readings sound normal to me, that's where mine are.
However the drop is definately not normal. Mine will dip a little, just for a second until everything adjusts to the load (or the bulbs warm up and draw less current, who cares which one =). I wonder if the brake lights and instrument clusters share a ground or something?
It almost sounds like the voltmeter is seeing the voltage drop on the wire for the taillights.
Stupid guess, but it's an idea?