Generally, I think the consensus is to put it into the line going to the actuator itself.
You will probably want to bypass the standard actuator control soleniod (front of block with 2 pipes and an electrical connector. Just pull the two pipes off and seal em up with a bolt in the end and a bit of tape.
Some people mount the new solenoid where the standard one is - others reckon it gets too hot there.
Cheers
GeeTee