Can you post a few pics of the headlamp switch wiring as it presently exists, as well as a general shot behind the dash where the factory switch should be located so we can see what you have to work with and whether there is anything abandoned behind the dash that you can still use? Restoring the OEM wiring won't be too difficult unless the PO stripped e/g out of the dash.
Installing a complete new harness is an expensive endeavor. Are there any other electrical issues that collectively assure a new harness is warranted?