2008 mercury mariner

I have a 2008 mercury mariner won't start i replaced the
starter only to have the new one do the same thing so i
took my old on and had it tested 3 different times and all 3
times said the same thing it's good so put my old one back
in to have it still do the problem of not starting but when o
put the screwdriver on the solenoid for bolts it starts right
up i changed the relays and still working what else could be
the problem