MyGray91Coupe

I have a 91 Coupe project car. I took the engine out so I could paint the engine bay. Put the motor back in and had to get a new alternator and battery. My problem is that the volt gauge inside the car doesn't show it keeping a charge and keeps going down like it is draining the battery. Made sure the two plugs going into the alternator were secure so I'm not sure what to do from there. I've been told that you need another ground other than going to the block and the negative side of the battery. Any suggestions?