Grumpy2, sorry to read of your heater woes. My +8 is of 85 vintage thus anything I type could be rubbish in relation to your heater system.
From your description of hot wiring and dimming lights relative to the heater fan motor circuit, that your thinking on the heater motor being seized may be correct... if there is any way you can get at the fan blades then trying to force them to move might just be enough to get it going again... However if you have put a large enough fuse in to the previously blown holder you could have burned out the motor windings, thus even with the mechanical aspect of the fan freed off it will still not run and just blow fuses..
if there is any sort of filtration on the air intake for the heater in the engine bay if you remove that it could improve the airflow through the heater rad while on the move..?
As for the water circulation, it seems likely that it will have a valve to cut of circulation through the heater box though whether it is a stand alone item in the engine bay of part of the heater box it`s self..I do not know. though the heater might just need to be bled due to an air lock in the rad...?
On my old Morgan the heater flaps are spring loaded with knobs on, thus no cable operation.
The heater valve is mounted in the engine bay and operated by cable which when operated either inhibits or allows the flow of water from the engine through the heater box, to determine whether there is flow or not through the heater with the engine running and water warming up, it could be a simple matter to feel both flow and return pipes for the heater to see if both are at similar temperatures, if one is hot and one not then it is possible you have a restriction, either the valve or the heater rad might have crud in it.... .
If your valve is u/s and of the remote mounted type (not part of the heater box) and is creating the problem, then you can remove it and replace it with a bit of copper pipe, which will allow flow but obviously no control over the flow, though at this time of year a permanently hot heater rad may be no issue..?
If the heated rad/core it`s self has an internal restriction then it is just possible to clear that without stripping the heater out... by identifying both flow and return heater pipes in the engine bay, disconnecting both, and connecting a garden hose to one end to flush out the heater rad.... then connect the garden hose to the end of the disconnected heater pipework to reverse flush the rad, if there is crud in the heater rad then you may see the water initially as brown in colour then turning clear, when the flow is reversed it may again run brown then clear, thus repeating the reversal process a few times may improve flow.... on reconnection you may need to bleed the heater by running the engine and disconnecting the return pipework till water flows out of it FROM the heater, connecting it back up while flow is still present, a bit messy but could solve the lack of flow issue if it exists...?
Do remember that if you have been messing about and loosing coolant in the process to then top up antifreeze ..!
Good luck in your fault finding..
