what does this go to on my 1970 cutlass?

 

Looks like an air conditioning compressor with the diode. It can be duplicated with Packard 56 terminal ends.
Although, re-examexamined, it appears to have a capacitor. Every GM car I've seen uses a diode.

 

I suggest that you use the wiring diagram in the shop manual to find its application. Look for a green wire (g) and a green wire with a white stripe (g/w).

 

Is this an original harness or an aftermarket replacement? Is this an A/C car or not? Is that connector part of the engine harness or the A/C harness?

If it's part of the engine harness that runs to the firewall connector, that is for the TCS solenoid. If it's part of the A/C harness that runs through a grommet in the firewall near the distributor, it's for the compressor clutch.

 

If it's part of the engine harness the object is a 150 ohm 5 watt resistor.