Your autolamp problem could be caused by the autolamp relay K331.  This is
located in the central junction box.  It could also be a problem with the
Generic Electronic Module.  Both of these devices are located under the dash on
the driver's side.  The relay plugs into the Central Junction Box.  Another
possibility is a problem with a connector or wiring.  Sometimes crip
connections fail.    If the problem is with the GEM, the new one needs to be
programmed.  That requires the special diagnostic tools.  Since the parking
lights turn on, the photocell and amplifier are okay.

I know this is a late post, but I have a solution for those who have this
problem in the future... I have a 2000.00 mercury sable ls premium with
autolamps that only turned on the parking lights when engaged with no
headlights. I went to my ford dealership and bought the following part
YF1Z-14A067-AA it's the central fuse/relax box. It doesn't include the GEM
module which is connected to the back of this fuse box. But your original
one will do fine. It the GEM module has one bolt and 2 clips on each side
pop that off and put it on the new fuse box and replace the 6 wiring
harasses and you should be good to go. I know it fixed my problem. My
central fuse/relay box's "NON"replacable relay went bad therefore it
didn't switch on the headlights. There are about 9 "NON" replacable relays
in that box which include your autowindow down, your defroster, autolamps,
hornstop, and a couple more are in there. So if they also go bad it's time
to replace that module. The part listed for $204.00. If you're anything
like me you'd get it replaced because the car isn't that old and honestly
I've gotten accustomed to the autolamps feature. Cheers and goodluck.

Tony aka mercurysableguy