I wish I could help, but I have gone all the way TO the heater box, but have never entirely removed it. If I were there to get my hands on it, maybe I could find the offending fastener.

Even someone who has had the box out numerous times may have trouble helping via the internet since there are so many fasteners and things that it's hard to know what you have and haven't removed.

One thing that comes to mind that might not be obvious is the clamps that hold the heater pipes to the firewall. They are pink plastic.

I'm sorry I can't help.

Good luck,