I found this wiring diagram
http://www.vanpeltsales.com/FH_web/F...wiring1953.jpg
And just realized that I confused myself when counting the wires in the car, I have 6 wires connected to the switch and 7 terminals (the brake light switch wire makes a jump and connects to 2 of the terminals).
Looking at this, the wire between the left brake light/left rear turn signal and the turn signal switch must be broken somewhere.