Some more thoughts-
1. Can NOX damage the engine if it is incorrectly used?
2. Are there conditions that might exist when you would want to lock out or turn off NOX regardless of driver demand?
3. Is your application related to continuous driving or simply a "drag race"?
4. What is the tach signal - pulses, voltage ...?
5. Does vehicle speed matter?
Sounds like something programmable that allows other conditions to be monitored would be best but that could be beyond your capabilities. If you wanted to wire something up you might proceed in this fashion:
A. Get yourself an indication of RPM that is a continuous voltage. The tach signal might be that already - or not.
B. Send the voltage that indicates RPM to two comparators - one to output hi when above 3000 rpm and one to output hi when below 5800 rpm. The set points can be adjustable.
C. Use a comparator to look at the TPS - to output high when wide open.
D. Use a 3 input AND gate - when all three outputs are high it goes hi - when hi the NOX is on.
E. I'd add timers so that if the AND gate goes hi then low momentarily, that the NOX valve wont toggle rapidly.
Personally I would not do it this way unless I really understood that the improper firing of the NOX would not damage the engine or cause surprises while driving. Note that an auto is extremely noisey in an electrical sense. Your circuitry should probably be built with considerable immunity. Again, this is a way but not really a recommendation. The cost of a commercial module ($180) seems small compared the the damage you might otherwise do.