[autoxerwgn]

I replaced a heater core on a customer's ti a long time ago but I remember that it's a pretty easy job to do. If I remember correctly, you need to remove the panel above the pedals and you should be able to see the tubes that run from the firewall area to the heater core. You can remove the tubes and then the heater core just slides out towards the driver's side of the dash. (Don't forget to drain the coolant first!)

You're lucky it's a ti, otherwise you would have to remove the entire dash and disassemble the heater box to access it!