This is a frustrating problem and you must be very careful while testing it. It sounds like the reel itself is bad but you cannot really check the reel with an ohmmeter. If you try to measure its resistance, even with the car battery unplugged, there is a very good chance that you will trigger it.

Was your replacement seatbelt new or used? If it was used I'm going to guess that it is bad but the only nondestructive way to tell is to replace it with a know good or use the test resistor.

Good luck with it.

...Lee