You have answered your own question!

The problem is almost certainly the exciter supply missing, which explains the random voltage, the reason being is when revved hard enough, it will excite the field winding's.

That's why I asked what was wrong with the old one as it was unlikely this was the issue.

You simply need to verify you have a switched +12 V supply to the 'D' terminal, I'll happy bet there isn't!

Martin