In a word-NO.
First of all 12 volts is way below the UL (USA) threshold for minimum shock value which is 36 VAC or 48 VDC.
Second, your car or truck is an isolated system provided by the rubber in your tires.
Thirdly, todays vehicles have surge protection built in so starting the vehicle does not cause thousands of volt to appear on the 12 volt power feeds as it did in the past century.
Worst that could happen is that you are working on your vehicle during a thunderstorm. If lightning hits your vehicle and you are standing next to it to work on it then it would be a shock by lightning. Avoid doing this if at all possible, as it could be fatal.