I would be very careful about connecting a lightbar into preexisting wiring unless you KNOW that there was enough extra current carrying capability. Lightbars (even LED) draw a lot of current. And lets face it...most MFG's design their wiring (and everything else) for "just enough" for the OEM system. With the cost of OEM wiring, I'd hate to burn any up. That's why I strongly encourage always going straight to a main power stud (Like the one into the cab, or in the fuse compartment. IMO, going directly to the batteries is not usually necessary.) Use a good inline fuse, and then a relay to turn it on with the other lights, if desired.
Doing this protects the original wiring from overload. Now, if you are replacing older lights with LED...thats fine, as you are actually decreasing the current load in most cases.