what are the pins for the wires? blue 74 and ? thanks

 

C1 74-yellow wire, and C1 80-black wire.

 

99-02 vortec truck it can depend on the harness and year and where the vehicle it came out of was built.

If you have a 99-02 truck harness with the flat 4 pin oxygen sensors, then the ETC low reference will be at C1 Blue-63. If you have the 4 pin square oxygen sensors, the ETC low reference will be at C1 Blue-41

99-02 LS1 F-body and 2003-07 Vortec truck harnesses, ETC low reference will be at C1 Blue-80

It doesn't matter what harness you have 99-07, the ETC signal wire is always on C1 Blue pin 74, yellow.