In one of the comments on this answer regarding ILS category IIIc operations (or, rather, the lack thereof), it's mentioned that
There is no [radio] guidance for taxiing except the pilot's vision.
And, in an answer to another ILS IIIc question, it is stated:
There is no provision in the ILS system to provide for an auto-taxi or runway vacate. IIIc is just zero-zero and roll-out control, not auto-vacate as the linked AvWeb article seems to suggest. Its a logical extension of the FAA definition, however auto-taxi is not part of the CAT IIIc definition or system requirements. The only requirement is to get the aircraft on the runway and stopped.
Right now there is no airport approved for IIIc approaches because the aircraft would just sit on the runway, it has no way of taxiing in. Until an augmentation system like GPS with WAAS or LAAS, or the "Ground-Based Augmentation System" is developed further, automatic taxi is not there.
There is no provision in the existing ILS signals that allow for an aircraft to vacate the runway.
Given that the ability to land is meaningless without the ability to taxi off the runway and to the gate (this is why, even though today's airliners are perfectly capable of IIIc landings, there are, in practice, no such landings, as no airport is equipped to taxi an aircraft in from a IIIc landing), why isn't there an "Instrument Taxiing System" to allow zero-visibility taxi?