I just looked at the CD. Sixto is correct. The blower speed signal goes from the PB Unit to the "Electronic Blower Control Unit" - R/W to a connector, then Y to the unit. Then there is a R wire to the blower from the unit, and a BU (blue) back to the unit (which I assume is ground).
So you need to press the various buttons and measure what you get on the R/W wire. If no change, the the PB unit is TU. If it does vary, then it is probably the control unit.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe