Electrical Problem

I have a 1990 Chevy G20 van. Both front fog lights, along with the left front sidemarker won't turn on unless they are blinking. The right front sidemarker works properly. All of the bulbs have been replaced and checked. Due to this, the left turn signal inside the van stays on unless I'm turning left. Then it works properly. Any suggestions as to what might be wrong would be appreciated. Thanks, Jimbo