How do the numbers 1-5 on the glow plug wiring harness correspond to engine cylinder numbers? Is number 1 GP in the front of the engine, or against the firewall? I have seen both sequences mentioned in postings and would like to get this right in my brain.

I happen to have my GP harness out at the moment for a re-wrap.
(84 300D) The # sequence starts at the front of the engine. A wiring diagram should show this as G1-G5. The glow plug wire colors, front to back(G1-G5), are- Blue, Purple, Red, Yellow and Green. Hope this helps.

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Diesel-guy