Glow plugs aside, I wonder about the general health of your car. My 81 SD with 290K miles would start without glow plug assist (bad relay) from March through October in San Jose, CA. Took up to 10 seconds of cranking at times but it would start.

Before you get new glow plugs, check all fuses including the 80 amp bare strip fuse atop the glow plug relay.

Sixto
95 S420
91 300SE
87 300SDL
83 300SD
81 300SD (sold)