Turn Signal Troubles

Richard are you working on the duster? In the front, it has the running light and the signal in the same fixture and using one socket. This requires a bulb with with 2 filaments. That means in your case a #1157 (clear) or a #1157A (amber). These bulbs have 2 pins on the base. The pins are offset vertically to help install them correctly, as the socket is similarely keyed. Its very difficult to install them wrong, but not impossible.Im going on the assumption that you didnt know this. This not a personal attack on you. Now the 1156 bulb IIRC is a single contact bulb. If you install a single contact bulb into a dual contact socket, its possible for the contact on the bottom of the bulb to straddle both contacts in the socket, leading to a crossfeed ;ie, power going to both circuits.
Now at the back, there is a similar situation, with the running light and brake light sharing one socket and requiring a dual contact(1157) bulb.
A single contact bulb is keyed differently than a dual contact bulb. This is supposed to prevent incorrect installations. Again, difficult but not impossible. Your Duster should have 1157s out back and 1157 or1157A in the front.The 1156s are for reverse.
The signal switch interrupts the brake circuit to the side you are signalling to and replaces it with the flasher circuit. Thats why I suspect that you have 2 problems as per my previous post. Its also possible that one or more bulbs at the rear are wrong or bridged filaments. Bridged filaments are rare. Again, just trying to be helpful.