Especially if you own an older Mercedes diesel with the relay under the dash near the brake arm. You should have a spare glow plug relay.

Cheap if you have a pick or pull car locally that is Mercedes diesel and older than 1981 probably. You can check the years for applications. I suspect the same relay from maybe 75 till 80. Verification is easily done on a parts listing site.

I have had a couple of these old relays just age and use fail. You can make a manual system with an old ford fender mounted starter relay as well. Did a quick parts check. It quickly looks like the original glow plug relay is a 1968 to 1976 specific part.