The trailer adapter can be located on either the left or right side panel in the cargo area. You need to remove the upper and lower trim panels around the hatch opening, then you can pull the panels back to expose what you need to get to.
You should start by just unplugging the adapter harness (there will be plugs on both sides to unplug) - it goes between the tail light harness and the body harness. Check if the lights work normally. If so, just replace the light adapter (plenty of options, check etrailer).
On my '99 it was on the left (driver's side), right here:
On the '97 I parted out, it was on the right side.
If that doesn't solve the problem, you most likely have a damaged flex PCB on your gauge cluster...
-Charlie
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'99 4Runner SR5 Auto -
4WD swapped
'89 Camry Alltrac LE 3S-GTE 5spd
'17 Chevy Volt Premier
'16 Honda Odyssey Elite
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