Not a silly question, no worries,
I used a cable that I had laying around in the garage that has 8 wires in it. Thinking back now I'd say you can use a regular cat5 ethernet cable if you can't find anything else.
To extend the harness I just cut the plug off about 6 inch inches and extended one wire at a time using the new cable. Once the one side was done I cut the new cable roughly two feet and spliced the plug back in to the other end one wire at a time.
Worked like a charm, good luck!