Glow plug lamp inop

Hello, i have a 76 300d. my glow plug light was not working so i check the plugs and #1 was open. i repalced all the glow plugs with bosch and checked them. all working and car starts fine. the glow plug light is still not working. i pull the cluster and replace the 191 bulb. still nothing. im leaning toward the glow plug thermostat switch but wanted to check. it seems every search i find is for people that changed to pencil plugs. and i cant locate a wire diagram either.

thanks for the help