How do you replace the thermostat on a 1987 mercury grand marquis?

First you need to remove dash pad and loosen the right side of dash, On engine side of firewall remove the bolt on left top corner of evap housing and the nut on top right, there is another nut under the case on the same stud, top right of evap. that nut is hidden there and could give you a headache as to why the case wont budge once you remove the two nuts on bottom of case in passenger compartment. there is a bolt under the wiper motor on the fire wall that aso needs to come out. Hopefully this helps you