Why does the drivers side heat work but the passenger side blows cold air in a 01 jag s-type v6?

It's more than likely a blend door that is broken. Look under the passenger side footwell and see if there is a black box towards the firewall. If there is then there is probably no way to get it out without removing the steering wheel, dashboard and discharging the a/c to access the airbox. I would recommend taking it to a mechanic in order to fix the problem. It is a very involved process.