300SDL means it's a 1986-1987 model. Unlike the earlier models with relays and an actual blower "resistor", the 1986+ cars are solid state with a transistor pack in the blower plenum. On the SDL's the key switch has a dedicated contact to the blower motor, if the transistor pack fails shorted, the blower will run when the key is on, but not when the key is off, and always at the same speed regardless of input from the CCU.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]