Just to summarize:

An 8pin pic running on INTRC OSC would perfectly fit with it's 6 I/O-pins.

1 pin for external switch / ignition detection
1 pin to drive the relay

leaves 4pins for "Config options"
that gives 16 different possibilities !

>>He can replace the 7805 coming out of his ignition switch
>>with a series resistor and a 5V zener.
this is absolutely correct, but
a simple voltage divider consisting of two resistors would do the job as well

Let Programmednew come back with his requirements and we will draw a working schematic for him.

The final board (including the relay) will fit in a matchbox.