https://www.ebay.co.uk/itm/Raspberry-Pi-DC-DC-Converter-Regulator-12V-Stepdown-to-5V-3A-15W-Module-Power/142752974294?epid=1274078478&hash=item213cbd85d6:g:JXQAAOSw5cNYcSC7
For the step down, or if you know the draw current you could add a simple resistor (make sure you get a resistor that can cope with the power dissipation.)
Step back up:
https://www.ebay.co.uk/itm/Mini-Micro-3-3V-3-6V-5V-to-12V-6W-DC-DC-Step-Up-Boost-Module-Converter-UK/113149182182?hash=item1a583780e6:g:QnwAAOSwnHZYTCXh
Switches: Depends on current draw, relays for high current, transistors for low current (mA).
Also try and fuse it like the car uses.