Good to hear from you again, but sorry for your troubles.
Based on what you're seeing with the trouble shooting you've already done, I'd say the obvious fault is a break or short somewhere along the wires from the switch to the fan motor.
That you are getting no power to the fan lead makes a grounding issue less likely to my thinking, but you should check that once you've got power to the fan lead.
You can check this with the fan motor reinstalled. You can take power on a lead directly from the fuse box and see if the motor runs.
If it does, that eliminates any grounding concern.
Clearly you're getting power from the fuse box to the switch and both the switch and motor appear to work properly. On some cars, there is a resistor in-line before the fan.
Make sure you've checked if you have power before that to the fan. You do, the resistor has some issue. You can also bypass it with power direct to the fan motor to check.
A car this old, I'd suspect and check for any bullet-&-barrel connectors to make sure they are tight, not corroded or broken. I've been chasing these kinds of issues on occasion over the 30 years I've owned my B.
In the past, I've had to replace them with newer crimp type bullet connectors, making for some unoriginal looking repairs that work well, but are not as the car was built.
Original style replica barrel and bullet connectors are now available individually or as kits from most of the catalogs. Generally it's the thin metal barrels that fail.
They're hidden within plastic tubes as either single (1 bullet connection at end) or double (2 at each end) so other than having fallen out, faults are hard to see or detect without getting hands-on.
The excerpt from the 67 MGB wiring diagram I've attached shows at least one in-line (circled). I know on my 74, there's at least (1) at the motor, but another elsewhere in the harness itself.
I hope this helps you. Keep in touch and let me know how you make out.
Safety Fast!
Art Isaacs