What's could cause this? on 2002 Hyundai Elantra

I currently have no interior lights or radio, scanners are unable to pick up codes, and check engine light is on. Recently replaced alternator and battery. Battery was drained after 2 days of use. Checked all fuses and all are good. Car is a 2002 Hyundai Elantra.

Asked by for the 2002 Hyundai Elantra
If the scan tools aren't communicating with the car, and the battery is going dead the next move is to charge the battery again with the cables disconnected, then when you connect them start measuring the parasitic drain that is killing the battery. Once you have that measurement, you can go to each fuse in the fuse block and measure the voltage across the fuses. Any fuse that has current flowing through it will have a voltage drop across it. (Old way to do this is to simply pull fuses and watch for the drain to be eliminated) Once you solve the drain, if necessary then we can talk about communication network diagnostics. I suspect you have a module that is failing and disrupting communication.
The fuses are all good, we have tested them all several times now. Repair shop charged and tested the battery and said it was defective. Battery has since been replaced and is working well so far. However we still don't have interior lights or radio. Scanners are still unable to communicate with the car.
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2 more answers , 1 more comment
if the battery went dead after two days then you should check your charging system and I would scan test for codes. check for power going to the alternator you could have a defective alternator or battery I would also recheck your fuses using a test light
We have attempted several different scanners, no of them can pick up codes making us believe that the computer may be bad. We have tested the fuses and they are good. We are replacing the battery today and the alternator seems to be working properly. I guess what we would like to know is if the computer is bad, if it could be causing the problems we are having. If not the computer, what else could be causing all these issues?