How do you expect that it will resist electrical noise? Focus ALL your attention in elimnating the offending noise.

Exact! It cannot be overemphasized, that the use of good filtering, shielding and grounding is real essential with any 8052 project! It's not only your project that will suffer from poor EMC measures, but it can also degrade system performance of manufactory installed car electronics!!

A good design begins with the PCB. Use at least double sided PCB containing solid ground plane. Connect all cables entering or leaving your board at ONE place at an edge of this board. Connect cable shields (if used) to ground at this point. Do also connect the ground pins of all used filters and protectors to this ground point. Use a metallic enclosure and connect it to ground, where the cables are penetrating enclosure. If it's possible, then connect all cable shields, filter and protector ground pins directly to enclosure rather than connecting them to PCB. This would even improve shielding and filtering performance. Then, interference has no chance to enter or leave your system!!

If you have implemented this, THEN you can think about some software features increasing redundance and reliability of your system. But NOT as a replacement for filtering and shielding hardware...

As Christian already mentioned DS1232, or MAX1232, can provide your application with a watchdog. By the way, their performance in protecting your system against negative effects of brown-out conditions and dips will 'harden' your application even much more! FLASH micros do not like these poor RC power on reset circuits. It's reported many times that Flash micros tend to loose their code, when not being properly reseted during brown-out conditions...

Good luck,
Kai