Ok so I got the impactor science to work. I was using a lander with a probe, landing it and then activating the seismometer. I would then switch vessels to the booster stage of the same rocket in orbit, and deorbit it and crash it. It also has a probe core. It seems to me the trick is to make sure the impactor is named as the "probe" and not the lander with the instruments. This can be done by renaming the lander's core before separation in orbit. Prior to this I had tried every combination of active vessel, not active vessel possible, no luck. Thanks for the quick advice everyone!