1992 Nissan Sentra I need help.

During the past 6 months I have gone through 4 alternators, and they keep failing. It starts when I notice the radio turn off while driving, then the car starts to stall and turn back on, finally it will stall and not turn back on at all. I have no clue what is causing the alternators to continually fail. The battery is good, the grounds seem fine, and for a while it works ok never exceeding 14 volts. I have an aftermarket sound system with woofers and an amplifier, but I installed a capacitor to protect the electrical system. I also have aftermarket fog lights installed. I did notice that the air conditioner is always working even when the switch is off because the lines int he engine compartment are always frosted over and icy. The only way to turn it off is by removing the fuse. Perhapes that is straining it.

I also noticed oil leaking from somewhere in the engine but I cannot locate it. The valve cover gasket was leaking but I replaced it and it stopped leaking from there but oil is still leaking everywhere. I noticed the alternaot covered in oil each time it failed could that be it?