It doesn't sound like the GP relay to me at this point. Sounds like an issue with the charging system. As recommended, find out why your battery is losing charge or not getting it back. Could be the voltage regulator or alternator. I've found many times that the culprit is corrosion at a terminal or in the end of a cable.

I say that I don't think its the GP relay because I've been able to start my 300SD with nearly 200k miles down to 10 degrees Fahrenheit without the relay. Mind you it took about 5 seconds of cranking and a bit of extra fuel, but it did start up quickly.

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