Speculation: on my 300SDL, there is a temperature switch that will not start the fan until the heat exchanger is up to given temp, to ensure no cold air in the face on start-up. Exception, if you push the Defrost button.

Your 190E may be different, but maybe the heat exchanger temp switch is bad, or maybe it got unplugged by you or your apprentice, fooling the system into thinking it's too cold to blow..

Best Regards,
Jim