The heat and AC blowers are separate. The AC blower is engaged when you turn the AC on. Sounds like you have a problem with one or more of the following:
- AC switchover valve may be sticking, not allowing current to the heater blower
- Heater blower fuse is blown
- Blower switch bad
- Heater blower bad (very bad news)
There are a lot of posts on the Vintage Forum on this.
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