I have a 2003 S40 with dual headlights that kept burning out every few months. I found out that this was fixed with an auxiliary wiring harness (one for each side). The dealer I purchased the part from would not help at all in getting me started...does anyone know if this part is ADDED to the existing wiring to provide additional resistance or does it REPLACE the existing harness?

 

Not sure but i would expect it to be a direct plug in replacement.
Check and compare the plastic end connector plugs. They should be compatible to facilitate a direct swap.