No crank

My friend has a 1996 dodge ram b2500 cargo van
we have replaced the starter and it still won't turn
over all it does is click when we turn the key so we
have replaced the battery ,starter relay, and we
checked all the ground wires and cables they are
all clean and in good shape so we took the new
starter out had it tested worked fine put it back in
and still would no turn over all it would do was
click so we went and got another starter and it did
the same thing we are lost has to what could be
rmwrong any help?