replacing detonation sensor on '90 300ZX

Nope, it's under the lower intake. You don't have to pull the heads but the upper and lower intake have to come off. You can't even get to the connector to clean it as that's ussually all that's wrong. On cars like that I'd replace the sensor AND the subharness just because it's such a bitch to get to.

I suppose you could "install" another sensor on the side of the engine block where you could get to it and run a new longer harness down to it? Otherwise it's off with the intake which is a PITA. The only good thing is you'll be able to change the PCV valves and clean the EGR pipes in the process! :-)