"behind the battery by removing the plastic false firewall section on passengers side"

You nailed it.

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Its the medium sized component. The big one is your "brain". The Smallest one that is partly red is your OVP.

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1989 300E 144K

I should also add that you will see 2 similar relays. One will say "Klima" on the top, that's not it (Klima is for the AC compressor), it's the other one, which should match closely what you bought, will say "6 ZYL" on it and also list an rpm speed, which is the safety cutout speed of the fuel pump relay.
Gilly

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Click here to see the items I have up for auction at EBay

Click here to see a photo album of my '62 Sprite Project Moneypit (Now Sold)

Pour example (This is a RHD model however the basics are the same)