Assuming a good starter and starter solenoid (you said it tested good)...
If the headlights turn on nice and bright and dont dim while key is in
start position, then you have a bad connection somewhere between the start
relay battery side terminal and the starter
a bad engine block ground. That usually goes straight from battery neg
post to engine
Both are heavy single wire cables. Could also be a bad start relay,
On the off chance it was missed, go under the car to the
starter solenoid. Check the ignition wire connector to be
sure it is both tight and clean. This connector has to be
squeezed to unlock and remove it from the spade connector on
the solenoid. They are bad about corroding. The fix is to
clean thoroughly and use pliers to crimp it a bit. If it is
badly corroded, you may have to replace the connector on the
wire. Thes is/was a TSB out on this problem for that
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