Starting Issue
 

Hello and thanks for any guidance! I have an 03 Jetta 1.8T that I bought used about a year ago. I was having starting issues a few months after I bought it. Wouldn't start at all and friend said it was alternator, replaced alternator and everything was fine for a couple weeks then same thing. Was told old alternator might have messed up battery and autozone said battery was bad, replaced battery and everything was fine for a couple months. Now problem is back it won't start and rarely even makes a sound but sometimes it will be very faint and build up like it's gonna start but never does. However, I can use my jump box and it starts right up everytime. This is actually what I've been doing and as you can imagine is getting very old. I've cleaned all connections at the battery as well as replaced connections and wire from alt to battery. When I roll up windows lights dim and radio turns off while rolling up windows. Immediately after turning car off power is almost gone, windows roll very slowly and only for a minute or so then nothing. I live in this car! That's not a figure of speech, I'm homeless and this vehicle is where I sleep. It's also my source of income as I drive for Instacart and do other app based jobs that require a vehicle. The only codes I get when scanning are P0420 which is O2 sensor but that has been on there since I purchased it. Please help!