MacBook Spacebar Not Working: Is it the Connector Cable or Keyboard?

The space bar on my MacBook all of a sudden stopped working properly. Sometimes it doesn't work at all, and other times I press it once and it stays pressed somehow and I get endless spaces.

I have tried re-setting the PRAM and SMC, and repair the disc and permissions with Disk Utility. I have popped off the spacebar key and cleaned under it. I have removed the keyboard and top case to make sure the upper case cable connecting the keyboard and trackpad to the logic board was firmly in place and that there was no dust or debris in the connectors themselves. Nothing has helped.

I assume at this point that I need to replace either the top case connector cable or the entire top case. The top cases are pretty pricey, especially given that this is such an old machine, so I would like to avoid purchasing a new one unless it is definitely necessary.

Any thoughts or advice about the most likely culprit here? The cable or the keyboard itself? Or any ideas about other ways I could try to fix this?

Thanks so much!

:)