Because your glow plug relay is sticking on, I don't think you can use the 'on' switch on the existing relay. You'll need to introduce a different relay controlled with a switch to the cabin. forcedinduction has a picture on here of such a system that he installed in his car activated by a push button (I think). Any adequate amperage relay should work. You will have to tie in the new relay to the existing wiring harness.
My view is that a manually controlled relay is superior to the OE automatic one. Afterglow can be achieved by continuing to hold the button.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13