Not able to left click. Firmware issue?

I have a Late 2013 15” MacBook Pro that won’t left click. Right clicking works just fine. Note that the physical clicking movement works fineThis happened randomly about a year ago. After getting a $500 estimate from several different places to replace the top case, I put it in a cupboard and got a new MacBook.

Fast forward to today… I’m bored. I picked up a new top case on eBay and installed it myself. But now, the issue persists.

The weird throng about this is I can’t left click with anything. External USB mouse, external Bluetooth mouse, nothing.

Ive noticed that it I can left click with an external mouse in the boot menu but only sometimes with the internal trackpad.

The other weird thing is if I disconnect the trackpad cable from the logic board, I can use an external mouse with no issues.

I suppose its possible both top cases have the same issue but I find that unlikely.

Here are the things I’ve tried:

Resetting SMC

Installing a new SSD

Re-installing MacOS

Installing all available updates

Can this be a firmware issue? If so, how can I fixit it?

Thanks in advance!