It doesn't hook directly to the W123 harness. Our harness has 3 spade connections, the AL129X only has two posts.

There are threads on how to, but basics: The thin (in my case blue) wire goes to the smaller post. Two new heavy gauge wires are made up and both go on the large post. You can choose to then run them to the stock junction box where battery and starter both have a lead, or you can run one to battery and one to starter directly.

Don't cut off the old plug, just put some nice electrical 'lube' on the contacts and tape it over, zip tie it out of the way for when you want to go back to stock....ya know for when these are worth a fortune .

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