Low Voltage Issues

I have a 93 Grand Cherokee Limited, I have been dealing with this voltage issue for a while now. Every time I am in the car when I have my defroster, wipers, or lights on my voltage drops drastically. Especially when I am idling at stoplights for example. I have turned every accessory on imaginable in the car while parked in the driveway and seen the voltage drop down to the lowest the volt gauge will show, (below 9 volts). I have also checked the power at the back of the alternator and battery and the voltage shows the same. I replaced the alternator, and had it tested after I replaced it to be sure it was working. Battery is also fairly new and in good condition. This gets very annoying as even though the car doesn't die, my wipers slow Down, lights dim, and blower motor slows down (which can get cold). I have also replaced the battery terminals for good measure. The really weird part of this is on cold mornings when I start the car it idles at about 14.1 exactly as it is supposed to, and drops just a little as I turn accessories on, (again as it is supposed to). But as the car warms up the voltage plummets. I am lost with this! Any ideas are extremely appreciated!