My personal advice (as a long time compulsive aux light worshipper): whatever you add, always take a relay fed from a reliable (aka fused) power source and triggered by your switch. This takes very little current from your existing circuit, thus reducing the probability of further instabilities.
This can sound a bit complicated at first, but it really isn't. A few wires to plug together (just like in software programming: do document all the steps ;-) and this eliminates most problems by design. Yet another case of "better safe than sorry" (don't ask ;-).