I changed 3 glow plugs on my 1982 Benz 240D. If the 4th plug is bad, can the new 3 plugs still work?

My 82 Benz 240D it is hard to start in the morning. I did change 3 glow plugs, but can't reach one of them, so one plug is old. The dash light stopped working after I changed 3 plugs. Will the new plugs still work?
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