Wanting to build an automotive grade solid state relay center
What you are describing is called a "high side switch".  It is simple enough to create one with a P-channel MOSFET.  Given that your inputs are "active low" - i.e., when "on" they are connected to ground, and when "off" they are at your input voltage (12V?), the schematic is even simpler.







simulate this circuit &ndash; Schematic created using CircuitLab

Q1 is kept normally off by R1 pulling the gate up to +12V.  When the switch is closed (or whatever "low" input you have is activated) the gate is pulled down through R2 (not strictly needed but good to have) and the MOSFET turns on.  That then allows power to flow through to Load +.

Diode D1 and C1 are to absorb any back EMF from an inductive load (e.g., solenoid) - they can be omitted from a non-inductive load, like a headlight.

I'll leave you to shop around for a suitable MOSFET and other components with the right voltage and current ratings for your application.