remove the wiper arms, raingaurd and possible the wiper motor (you can work around it but it's a pain) then you wanna loosen the strap the holds down the blower motor/heater core box (dead center against firewall) remove all vacuum lines and electrical i.e. blower motore power etc. After removing metal strap that goes across the center horizontally (one phillips head) remove the send and return lines for the heater core ( i think there on the drivers side) check for anything else attached I may have forgotten and start wiggling the whole unit as to break it free from the plumbing inside the vehicle, some say you need to pull you inside knee panels etc so pull the interior air hoses off the blower/core unit but i've found this to be more time consuming however you do need acces when you put it back in. Anyway, wiggle it out CAREFULLY especially in this weather! once out pull out the fan again CAREFULLY! then you should be able to split the unit into two pieces and the core should be accesable/replaceable. Good luck, fella's correct me if/where i'm wrong....

2006 VW Passat 2.0 TDI 6 spd
1994 Volvo 440 1.8
1987 Grand Wagoneer
3 spd mens bicycle