Contact an expert who does mold and mildew removal. If it's black mold it will smear like charcoal.

If it's just mildew, the fun way is to load up the truck and take a vacation. Get on I-40 and drive to Texas/Oklahoma in the summertime. Low humidity and plenty of heat. Roll down the windows. The heat will kill the mildew and you may be able to lightly brush it off. When you get back home, park the truck in a dry covered area or garage. The not so fun way is to replace the headliner. A lot of sweating and cussing. Another option is to leave it alone and call it patina.

"Adding CFM to a truck will only help at engine speeds you don't want to use."
"I found there was nothing to gain beyond 400 CFM."