If you're talking about physically, you can put it anywhere you want. I put mine on the firewall between the brake and clutch master cylinders. If you mean where does it hook up, then you tap into the fuel return line. Theres a hose that runs from the J tube on the block (the tube/cover that covers the old mechanical fuel pump opening) and goes to the fuel return pipe just under the firewall. Buy some hose, and run the hose from the J tube to the rrfpr, then run it from the rrfpr to the fuel return pipe.
Check out my old site: http://www.geocities.com/aarongarney/5mgte.html There is a picture showing the connections Racefiend mentioned. Also, make sure to get high pressure EFI (electronic fuel injection) hose, not just standard fuel line. Hope this helps.