The dimmer switch does switch the function of the T handle but without hydraulic controlled wings it may not do anything for your set up. Do you have extra hydraulic ports on the front of the cab that are not in use? If you do they are what would be attached to the movable wings if you had them.
Do you have the hydraulic quick disconnect for the blade or is it a manual lever mounted on the blade? The 200D I still have has the foot switch(for the wings) and then it has a small toggle switch mounted at the base of the steering column, just above the left foot that switches the T handle to control the hydraulic quick connect on the blade. you should see hoses going to a single cylinder on the blade for that purpose if it came with your cat.
The knob you are describing could be the emergency brake button that when pulled(or pushed in I don't remember) will activate the parking brakes and bring the cat to an abrupt stop. Not all the 200Ds had this feature. It would be just to the left of the forward T handle
Or it could be the knob that when turned one way increases the coolant flow to the cabin heater and when turned the other way shut off the coolant flow( heat control knob). That knob is to the left and behind the rear T handle.
In any case when you tilt the cab you can easily see if it is plumbed into your parking brake handle or is plumbed into your coolant system.
I actually have an extra copy of the manuals I had made years ago and can mail it to you. It has copies of trouble shooting manuals for all the electronics and vehicle drive components which is invaluable to adjusting and fixing the 200Ds. It also has two different owner/operator manuals that are not 100% complete but most of what you need to know is in them. They are from different years so some things may not apply to your cat. PM me your mailing address and I'll try and get it out to you in the next couple of days.
Randy