Try posting your complete circuit, so we can see what might be happening. Are you also sure that your existing 5V supply is actually a 5V stabilised supply?, historically cars used to use crude pulsed 12V DC rails to feed the instruments, not an actual 5V supply.
Presumably though a modern vehicle would have a proper regulator to feed it's electronics?.
Personally I would rather feed my PIC from it's own regulator circuit fed from 12V, I'd be MUCH happier knowing exactly what I'm feeding it.