virtually everything going to the back of the car, routes thru the turn switch.
You could very well have a turn switch issue here.
For the brake lights - power goes from the brake switch to the turn switch, and with the signal in the neutral position, it is then split to two separate lines that run back to the rear lamps (batttery power)
For the front signals - the power (ignition buss) goes thru the signal flasher and is then switched to either the left or right FRONT signal.
When the left signal is activated, the switch connects the left FRONT to the flasher--and at the same time, disconnects the left rear from the brake switch and connects it to the left front---thru the flasher.
The right brake signal is still getting power from the brake switch (battery buss) if the brakes are applied.
All kinds of things going on with that switch---so, if the guy did not replace it, and only patched the harness, there is a possibility that a different wire broke.
IF your car has a tilt column----the wiring in the column WILL eventuall break
(2x on my 79 C-10)
A new turn switch should not be all that pricey----but can be (if tilt) a
ROYAL pita to change