You have to give us your WHOLE schematic, there's no obvious reason why it shouldn't work.

If you supply line is pure crap, it will give you crap. In car it's usual. Tie ALL unused pin to somewhere first. ALL floating i/o in car apps (and everywhere anyways, trust me or not i don't care)... is often a killer situation.

Now if one of your input is MCLR and you didn't use the right voltage divider, your PIC will fall into programming mode... too bad.