Need help with VR
 

i had a 85 mustang and did an engine swap in to a 91. the 85 had a external VR and the 91 VR is on the alternator. how would i go about hooking the alternator out of the 85 into the 91. i tried hooking the wires up the same way the 85 was but the wire are different. the first time i did it the battery light came on and i changed the wire around and the light went off. the problem now is that when i turn the light on the volt gauge drop and the rpm needle jumps. any help on the subject would be great..