My Vitesse had a poor heater, I took all of it apart, lubricated the fan motor and put a new control valve and flushed the matrix. Then tested the switch and found it was really intermittent. Found a NOS one on e bay for £15 and fitted that, bingo, fan much faster and heater warmer.
Not that it matters much in a CV but it did help when I was driving it in January with the roof down at -1 degrees.
S