Auto dim mirror wiring
 

I've got the auto dim mirror and apparently the seller can also let me have the wiring loom to go with it. The question is this: does this loom simply plug into the A Pillar plug and have a connection at the other end for the existing interior light as well or do I still need to transplant the 2 (or 3) wires from the mirror loom to the existing one and leave the interior light untouched? I've looked at the How Tos but my head started spinning before I'd come to a satisfactory conclusion.