I had mine under the dash upside down no problems as well. From what I recall, MSD does not want them upside down because there are drain points on the bottom of the unit that will allow moisture to escape if the unit gets wet and in the upside down orientation there is no way for moisture to escape. The is more critical if the MSD unit is mounted in the engine compartment or someplace else where the unit could be exposed to water. If the MSD unit gets exposed to water mounted upside down under the dash, the unit getting wet and not being able to drain is probably going to be the least of your worries
