I put a new battery in and know the ac and heater control show all the info but nothing works

Sounds more like a lose connection in a wiring harness . Just replacing parts isn't the way to fix it . Taking your vehicle to a qualified repair shop that has a professional scan tool an checking for DTC's - diagnostic trouble codes , an viewing data inputs to the different control modules is the way . You probably don't know about serial data networks an how they work ? All the gauge info comes from the engine computer over data bus. When you see -------- this the info got lost or the ECM/PCM can't send the info.

Info I have leads to alternator but have video which show how you can test alternator. One way shows to pull the wire connector off alternator the one which is plugged in, see if the speedometer is corrected. Diodes in alternator could have a problem you can check at battery with AC voltage check on volt meter. Output speed sensor on transmission reads in AC voltage and diodes in alternator could be putting out to much AC voltage wave. I know you said you put in a new alternator, you may want to take back to dealer and have retested.

https://www.youtube.com/watch?v=4llG_Hb6178
https://www.youtube.com/watch?v=Nhc5VqlUrY0

https://www.youtube.com/watch?v=MGaHaIXZioA
https://www.chryslerforum.com/forum/chrysler-pacifica-15/surging-idle-speedometer-showing-speed-when-stopped-13761/page2/

https://www.youtube.com/watch?v=J8F5uCCOdH4


https://www.youtube.com/watch?v=yxq9ByC-Sd0

What year is your vehicle? It could be a control cluster, or "brain" as it is often called, which is directly behind the AC?Heater control knob. This would require removing much of your dashboard which is NOT a enjoyable experience! Peace. God bless.

SOUNDS TO BE A BAD CONTROL SPEED SWICH

If you have a drain on your battery , it would not take 18 months for the battery to go dead . What do you mean The battery gauge in the dash shows only about 1/2 power, When ? When the vehicle is running ? Not charging ! The battery gauge as you call it has nothing to do with the Battery . That's showing how much the charging system is putting .The volt's indicator is controlled by the PCM - engine computer . With the engine running should show 14.1 volts .if your only seeing 7 the charging system isn't working. The alternator is controlled by the PCM as well ! Have it check by a GM dealer .

you havent put the fan belt back on by the sound of it or the pulley you put on has the rubber insert broke and its not turning the belt ,have a good look

Looking at a wiring diagram an testing the electrical circuit with a DVOM - digital volt ohm meter is how one would diagnose this . Video's on youtube showing how to do it .
Electric Testing Techniques You Need to Know You can buy a cheap DVOM at a auto parts store ..Do you know what a wiring diagram is ? How to read one ? Free wiring diagrams here http://www.bbbind.com/free_tsb.html Enter vehicle info. year , make , model an engine size . Under system click on HVAC , then under subsystem HVAC controls will pop up. Click the search button then the fourth blue link. Blower motor circuit . Watch the video ,then test for where the B+ voltage or ground is missing .

1. Check the fuse to the fan
2. Have a look at https://www.youtube.com/watch?v=ELg0H5Dr014 and replace the sensor.

(I'm not sure what the problem is with the ac/heater: I understand that the blower worked but, for some reason you replaced the resistor, and now it doesn't...)

Sounds like 2 probloms there is a bad alternator so the battery is not being charged, and the fuse for the heater/ac fan is bad

Change the 15amp fuse in the kicker fuse panel labeled cig. My stopped working for the same reason.