Good luck. I’ve researched and looked at old Ford promo ad pics with no consensus that I found. Radios were added by dealers and owners, so both sides utilized. It appears to me drivers side most popular, but not necessarily accurate. Okie-Wan, all knowing 66 detail expert, hopefully will chime in with the answer. But 66 rigs, by their nature of build, have many mysteries as to “original” build or part. Like visors, mirrors, etc. Great info on several CB 66 threads. Have fun !