what's this for??

This cable is supposed to be connected to the left side of the heat blend door actuator, but it seems to be giving my actuator issues when it is attached. Anybody know it's function??

Based on your photo, and what looks like a grommet going through the body, this might be the cable that connects to the valve that allows coolant to enter the heater matrix. You should be able to see it if you peer through the grill and see if it lines up with where the cable is going. Your valve may be gunked up and is struggling to be opened/closed by the actuator. I recently flushed my rear heater matrix, and at the same time flushed out and cleaned the valve body, which had a lot of crud in it.

Awesome, you’re absolutely right! And the cable and valve are pretty rusty and jammed up. I worked it back and forth and saturated it as best I could with WD40 and it helped somewhat, but I might see about replacing at least the cable and doing some rust removal on the valve itself. Many, many thanks!