Starter Relay Help

I just finished putting a new crate engine in my 94 GT. When I went to start it nothing happened.
I checked the voltage at the starter relay. One terminal has 12 volts all the time. Another terminal has 12 volts when the key is turned to the start position if the transmission is in neutral or park. That terminal has no voltage when the transmission is in drive or reverse when the key is in the start position.
I took the cover off the relay and pluged it in. With the trans in park, turn the key to start the relay doesn't activate. If I pull it out and supply 12 volts through the coil the relay works fine.
Looking at the wiring diagram, it looks like a very simple circuit and it looks like everything should work.
What am I mising?

Thanks for the feedback.
When the car didn't start, I disconnected the wire to the solenoid and checked the voltage when the key was turned to the start position. It had no voltage. That's when I went back to the starter relay in the fuse box. When I cannect a voltmeter to the terminal for the relay and turn the key to start, I get 12.09 volts. When I plug the relay in and turn the key the relay doesn't switch on.
Since no one told me about some special circuit I didn't know about I went back and plugged a spade terminal into the relay slot, connected that and the volt meter together and turned the key. Low and behold the relay worked. I guess that the terinal slot for the realy is my problem. The relay must not be making good contact.
I was going nuts for a while there but I think I can get it now.
Thanks everyone for your help.