1994 mustang gt electrical problems

If the alt checked out I can't see how it is the voltage regulator. You said you rebuilt the engine. This means you took it out. Have you checked ALL the grounds?

im pretty sure i have. there is one going from the frame to the motor mount and there is another one from the engine block to the battery and then there is alternator and the stater. should there be more?

Make sure all engine grounds are attached. Make sure all connections to the alternator are good.( if they look melted/burned , replace them ) Make sure all fuses and fuse links are good. On this style of alternator , the regulator is built into the alternator !! Check to make sure battery connections are good ( not corroded ) I hope this helps out !!

Your alternator should have a large wire bolted on, this wire should have battery power all the time.There should be a 3-wire plug and a single wire plug. On the 3-wire plug the white wire with black stripe goes to single wire connector that plugs into alternator. the center wire should be yellow with white stripe,it comes from 20a fuse in engine compartment. The light green wire with red stripe comes from dash light which is powered from #18 20a fuse in instrument panel fuse box. You would have to use a volt meter to check for power or a test light, The light green wire that comes from dash has power with key on, If you use a test light it might be dim. If no power on wires check fuses