1997 Mercury grand Marquis 4.6l starting issues

I had just acquired a 1997 Grand Marquis. I took it
to get it snogged... Past flying colors.  Drove it
around for 3 days... No problem.  Then we get
home and go to start it again and nothing.  
Sounded like a weak crank on the starter.  Lights
will turn on and all but not enough for the starter.  
Replace the starter and same thing. Had them test
my core starter and it tested fine along with the
battery.  I have new wire to the starter from the
battery and ran another ground to the engine.  
Same thing... I'm stumped