2000 mercury marquis wanders all over road constantly have to touch steering then over corrects.

Are tires low on air pressure, old or junk otherwise, if not then may need tie-rods/ends, Pit-man arms, steering linkages, ball joints or bushings.

Loose lug nuts. Possibly just need to get it on a rack to see what is worn out, there is a lot of front end parts that can be worn.

Are tires over-inflated, bald or wearing un-even?

Worn front end components. Take it to a quality alignment / suspension shop for evaluation.

I’ve replaced everything including the gear box and still have the wandering issue.