Load dumping occurs when a large load is suddenly disconnected from the electrical system. The alternator, working hard to keep up with the load can not react instantly when the load is "dumped" and will put out a huge spike into the system. Sometimes as high as 60 volts.
Circuits should be designed to protect from reverse polarity too.
I believe there is an SAE guideline for automotive circuits.
Here is Hella's version of a S/S relay:
Amazon.com: Hella H41773001 Solid State Ceramic 32 Amp SPST Mini Relay: Automotive
Note that this a SPST relay. It might be a challenge to make a SPDT version.