WTF is wrong with my car (95 with s13 blacktop)

[ATLnismofreak]

hey guys, so im still trying to get my sr to run...ill charge up the battery, start the car, then watch the voltage drop like panties at the prom! i just tried replacing the alternator, but its still not holding power (im not sure if the new one is good, but im gonna have the original tested tomorrow and maybe rebuilt)...other than the alternator what could cause this rediculous drop in voltage

ps i have a miata battery but im pretty sure this isnt the problem

Also, what does the bigger plug that runs along the side of the passenger side door go to? There are two plugs that come up from over there, i found where one goes but not the other one? (ill get a picture of the plug tomorrow when the suns out)