Did you look in the 1970 Mustang Electrical Assembly Manual? Your picture shows a blue wire with a red tracer that goes to a two pin rubber connector with the blue-red wire "protected" by a portion of the connector; and a black wire "exposed" in the connector. The "protected" wires in connectors used by Ford are usually hot, 12VDC; the black "exposed" wires are usually grounds. Ford usually keeps the same color codes for all years (my 1999 E250 has many of the same color coded wires). The wiring diagrams in the Assembly Manual shows a blue-red wire used in the light dimmer circuit and identified as "wire 19 (A, B, C, etc)". I found one that fits the blue-red and black in a common connector. It's for the ash tray illumination circuit.
There may be more.
Jim