Why won't this thing turn on?

I bought this laptop to refurbish it. I got a new display, top palm rest along with keyboard, aftermarket battery, and aftermarket charger.

My first time fully taking it apart and replacing damaged parts, it worked. I was able to login and take care of some stuff.

I didn’t adhere to the battery and I also forgot to install the backlighting cable. I went to finish it off by fully installing the battery and reconnecting that wire. Now, the MacBook will not boot.

All I get is the fans spinning when it’s plugged in, nothing at all when it’s not plugged in. I got it to work for a second but it booted up playing the sound and the cut out again and did the same thing on repeat.

Now I can’t even get it to play the boot sound anymore. I have taken it apart again and reassembled it. Still the same thing.

When it’s plugged in, I can hear the fans going higher in RPM and then lower like in a pattern. Not sure what’s going on. I already tried resetting SMC and trying with and without battery. If it’s not plugged in, the laptop does nothing at all.

Please help LMAO! I’m going nuts.