The risk on using an RPM switch is that you can fry the VTEC selenoid plus you're not getting the fuel maps that the engine needs when going into VTEC- best bet is to get the ECU for the EX, Im not sure about which wire goes were in the '98 ECU but here is what I got for the '95 Ex, at least you can have and idea and you can search it one the Net...
"ok.......now on to wiring vtec
you need the plug on the vtec solenoid with the cut wires......there should
be 2 wires from the bottom harness......and 1 wire from the spool top
harness......
1st harnes( 2 wire one).....
1 blue wire- oil pressure sensor
1 black wire- ground
extend the black wire and run it straight to a good solid GROUND
blue wire should be extended through the fire wall straight to D6 on the ecu
2nd harness(1 wire one)
1 green w/yellow wire- vtec solenoid
run a wire from the green w/ yellow wire all the way through the fire wall
to A4 of the ecu
now....thats 1 wire going to ground....and 2 wires going to the
ecu.........now...........there should be NO......I repeat NO check engine
light......unless you didn't hook up vtec rite............."