I recommend never using "used" electrical parts on any vehicle,alway get one from a par ts store,you never know what condition the used one is in,or how long it will work,or in your case, what might be missing from the used part.
Replace voltage regulator. About $15-$25 part. 5-10 minutes to replace. A simple DIY job actually. Always use a new voltage regulator. It is strange the new one didnt come with one as they usually do, at least the OEM ones.