The 66's have two "neutral safety" switches. The one that controls the transmission, (locks out except in Park and Neutral, is mounted on the transmission at the selector lever.
The second is (should be) mounted on top of the steering column, about a foot from the firewall. It is the same switch as the 64's and 5's use for the neutral lockout. Except, on the '66's there are no electrical connections to this switch. Just two vacuum lines. You line it up so the arm on the shift rod contacts the spring-loaded tab on the vacuum
part of the switch, When the car is in Park, the arm on the shift rod should push the tab on the switch over as far as it will go. This closes the valve for no vacuum. When you move the shift lever from Park to drive, the arm on the shift rod rotates and allows the spring-loaded tab to move the switch over, opening the valve to allow vacuum to release the em brake.
If you do not have the steering column mounted switch, or vacuum lines, someone must have removed them and wasn't worried about having the auto release of the em brake, - just either didn't use the em brake, or used the manual release on the foot pedal mechanism which is a bit difficult to do on a repeated basis.
If you go to the Bird Nest or other T-bird
parts sites, you can see the difference between the two switches. They will be marked "for '64 and '65, (a white plastic body)and the other flatter metal switch will be marked "for '66 C6 transmissions."