For my dual 12" fans, I tapped into the Low Fan relay. I didn't follow those 240sx.org instructions but my install sounds the same. There's 4 slots on the top side. With a multimeter, see which slots provide power w/ ignition key in the OFF position. Then, check which provide power with the key in the ACC position. The latter is the one you want to tap into (fan on only with ACC or car on).

Pop the relay out, run the wire from undernreath the fuse box, tap into wire, solder, reassemble.

What's the amperage load of that fan of yours? Remember that the relay is rated for 30amps so if you have the OEM fan still, make sure you don't overload it.

I'll be in town soon, I'll show you if you don't want to mess w/ it.

Jesse

PS - I hope you have a KA b/c the above is not true on the SRs I've worked on.