How do I wire my Furnace Fan to kick in and maintain humidity levels?

Hi,

I want to install a thermostat/humidistat combo that will allow for my furnace fan to kick in when the humidity drops.

In the summer, the heat will never run, and thus my humidifier will never run since currently, that is the only way to run the humidifier.

Q: Is it possible to wire up my furnace fan, to kick in only when the humidity in the house gets below the desired setting? - I do not desire to have the fan run 24/7, only when humidity needs a boost.

What parts are required to do this?
Please recommend which thermostat will perform this job.

Details so far:

At my thermostat location:
- there is a red wire, white wire, two black wires, and another white wire.

I have a GeneralAire 1042 humidifier.
Furnace is Vancouver made Airco 8300 Turbo (25 years old).

The 25 year old humidistat upstairs - was connected to both black wires. It no longer works, previous owners say the white wire is broken somewhere in the wall. New humidistat is in the furnace room. I might have to fish a new wire, no problem.

Thanks!