big job. You need to take out the entire dashboard. Start with the handbrake surround console, then the centre console, then the vents/radio/heater controls, then take of the steering wheel and remove the instrument panel. Glove box and the panel under the steering wheel next, then its only two bolts on either side of the dash - lift out the entire dash. The heater assembly is black, and the matrix is inside it.

it takes about two days, if you're going slowly like i was. Just be careful about connectors behind the instruments etc. Most have lock tabs to pull up.

good luck!