Is someone able to point me into the right direction for instructions on converting my 1948 Merc M47 pickup from 6volt to 12volt and where I can obtain a quality 12v wiring harness from?
There is not any reason to change the wiring harness unless it is in poor condition.
For the sake of originality you can obtain an original harness from various vendors.
I'm not going to get into the 6 versus 12 debate but I used a Rebel wire 9+3 kit when I rewired my '39 p/u and changed over to 12 volt. Excellent product and from all accounts they have great tech support but I didn't need it. USA made as well. http://rebelwire.com/rebel-wire-products.php?cat=Wire Kits
I converted my M 47 to 12 volt. Once again, very easy and not too expensive. I used Speedway's 12 circuit kit, instructions are easy to follow. I've used that kit on 5 trucks so far, it takes about a day to do the job. Converting or not converting the gauges is your hardest decision.