Haynes has the wiring diagram for the L-jet fuel injection. (So does Pelican; check the "914 wiring diagrams" link on the left side of the main pages.) The wires on the diagram are numbered; if you look at the wires in the FI wiring harness you should be able to find numbers printed on the white wires near the ECU end of them. You can use a voltmeter or continuity tester to find the other end of each wire, and the diagram will tell you which wire number goes to which injector. (I think.)
Jeff Bowlsby knows a whole lot about the fuel injection wiring harnesses on these cars.
Replacement Porsche 914 Wiring Harnesses I think he may have a 1.8 diagram in a more traditional layout, which may be easier to read.
--DD