Check fuses and power supply Some Amps still have screw in glass
fuses.  

Next there should be a gain adjustment.  If everything else is ok,
with the radio on try adjusting that,  if it's working you'll notice.

Then (or perhaps even first) check for power at your sub with the
stereo on.,  Check to see if there's a defective capacitor  (not sure
if the Lincoln's use them or not but.....

Let us know how it goes.