Where is the best place to splice? Temp sensor or the back of the factory guage?

What kind of gauge are you trying to install? Oil temp, water temp?If it is electrical (which is what it sounds like) then you should be using the sender that comes with the gauge, or goes with that specific gauge if it is seperate. The aftermarket gauge most likely uses a different signal (resistance) than the aftermarket gauge so if it were to work the readings may be off somewhat.

Thanks!