factory switch is located on the drivers side of the engine block below the fuel filter canister.
for adding an aftermarket one, you can go with either of the 2 plugs on top of the filter head, or from the 2 accessible plugs in the drivers side of the engine block [in the same drilling as the oem switch] those ones need an adapter to go from m10x1. 0 to something [to adapt to your sensor or line for aftermarket gauge] the ones on the filter head are standard 1/8" npt