I’m stuck with a Mark VIII (named Christine) that has been almost totaly rebuilt…piece by piece. My latest…and LAST problem, is the steering. After changing the rack, upper ball joints, power steering pump, tie rods and struts (bye bye airbags) the steering is still heavy at low speeds. What can be left to replace? Everything, including the pump, checks out o.k.
What about low tire air pressure, alignment, or dry lower ball joints (which is where the steering knuckle rotates).
With that many changes the car should go onto the alignment rack no doubt.
Maybe it has a stuck steering stabilizer. It’s the thing that looks like a shock absorber and has no hydraulic lines.
Marks do not use a steering stabilizer.
Maybe the rack you put in is no better than the rack you took out, very common with “rebuilt” parts…
Does the Mark VIII have an electronic speed-sensitive power steering system? I know it was an option on the MN-12 Tbirds and Cougars.
Yes, it has electronically controlled variable assist steering; electronic variable orifice.
It’s controlled by the air suspension module and of course requires a specialized scanner to get any codes that may exist.
The OP might consider checking the fuse in the underhood fuse box. There is a 10 or 15 amp fuse providing power.
Maybe the problem is a blown fuse… 
When you converted the airbags to coil springs what did you do with the air suspension module? There are several connectors to the module and only one must be unplugged (the gray one if I remember correctly). If you unplugged both the gray and the black then you may have killed your variable assist steering.