Dana,
Jeff has the correct procedure, but I would suggest that you release the wiper linkage to get this job done. It is not absolutely necessary, but I found that it was easier that way. If your defroster distributor is as brittle as most are, then you need to be very careful not to mess it up. Releasing the linkage lets you easily move (or remove) the distributor so that you have plenty of clearance when you slide the heater box to the passenger footwell. And don't even think about removing the defroster box on the driver (LHD) side, they built the entire car around it.
Best,
g
'64 230sl, fully sorted out...ooops, spoke too soon