I am currently in the midst of a similar task, also with a 1980 300td, coincidentally.
You say the in-car sensor is faulty, do you mean it is dead or that it isn't providing the proper signal? My
understanding is a loss of input from any sensor in the chain will prevent the system from working.
On the blower: I was recently reminded that apparently, a blower motor with well worn brushes may not come on except on defrost, because only on that setting does it get a full jolt of power (max speed). (I am passing on what was reported to me, I am not the owner of that factoid
.)
Have you tried applying 12v directly to your blower?