Electronic – Sense Automotive 12V Circuit from 3.3V microcontroller safely

How about just using some transistors? Get an NPN transistor and put like a 10k or 100k current limiting resistor between the base and your input signal. Connect the emitter to ground. Connect the collector to an I/O pin and then add a 10k pull up resistor. You can get transistor array chips in various places that will do this, and you can support voltages much higher than 12 volts so long as you use a large enough current limiting resistor. As is, the threshold will be around 0.7 volts. Need a larger threshold? Use a voltage divider instead. A 100k series resistor with a 100k resistor to GND would make the threshold about 1.4 volts.

Alternatively, you might be able to get away with just voltage dividers on the inputs. A 10k series and a 3.9 k shunt resistor would get you pretty close to 3.3 volts with 12 volts coming in. Your microcontroller should have clamping diodes internally, you just need to make sure you don't accidentally overload those. If you're really worried about it, there are diode array chips that you can get that will either tie everything to ground with Zener diodes of the proper voltage, or clamp the inputs to power and ground.

If this circuit diagram for the transmitter is correct, you don't need to level-shift your output:

The TXD pin is the input to the base of an NPN transistor, so all that's necessary is enough voltage to turn on the transistor. Your 3.3v output from the RPi should work fine.