switch control and air conditionning problem

Hi all.

My VW passat GL 1991, it have problem is control level from 1 to 3 it don't working but the air conditionning lighter on and blower air motor not work and then I move swicher control to munber 4 level is blower motor working .??? it stranger. Have someone known this problem? Please, help me tracking this problem

Thankyour.