Designing circuit board
For the past few weeks, I've been looking into what to do with the electronics, and decided that the PIC microcontroller would be the right way.
I used Eagle PCB for the schematic design and subsequent board layout. I then created a single layer board (although I had to use some jumpers to link some sections together).
Since I had 15 inputs and outputs, I chose the PIC18F2331 controller, which is available as a 28-pin DIL chip, and can be programmed with the PICStart programmer I've got (after you've soldered some components to it). I'd used one for an earlier project, and it was now spare (and I've also got another one).
I made the design a generic input/output circuit controller, with 12 inputs and 12 open-collector outputs (with diode protection for relays).
I used some surface-mount resistors to reduce the number of holes I had to drill (it's very time consuming to drill holes). I would've use SMT for the diodes and transistors, but I knew I didn't have enough of them to complete it.
Making the PCB
I won't go into the details of how to make a PCB, as there are lots of sites on the Internet that describe it.
This is what the finished PCB looked like:
Populating the PCB
It took a couple of hours to find and solder all the bits on (for some reason, the SMT resistors had been put away from my usual store!).
I did a quick electrical test, and found out that it consumes 4mA using the PIC I'd used on the other project (and it hadn't been reprogrammed, so it's probably using more current than it needs to).
Tomorrow, I'll start programming the PIC, and wire it to the steering column wires.