Have you tried running your ground wire all the way back to the negative terminal on your battery? This may help.
Also, an input voltage of 15V max is a bit on the edge for a car PSU. This may be part of the problem. I have seen spikes of 16-18V under various conditions. I wouldn't recommend the 1N6273A since it's breakdown voltage is 12V. You'll probably burn it out quickly if you're using it on the input to the PSU since there's nothing to limit the current into it. If you are trying to add some sort of overvoltage protection on the input, you need something that limits around 16-18V. However, you can't use this as a "pre-regulator" to regulate your regulator..... . These "mosorb" type zeners are designed to take very large, very SHORT transients. They are not designed to be used as a voltage regulator for high power dissipation.
Hope this help! Good luck!