12v Automotive voltage as input to Arduino: Optos or SSR?

i working on automotive project. want use 12vdc outputs car (which means can peak @ around what....15vdc?), meaning when circuit made. want read outputs inputs arduino.  or put way, when switch on headlights, want know switched on reading 12vdc @ headlight input arduino pin.

my options, research either use opto (nice article here of needing 12vdc input):
http://electronics.stackexchange.com/questions/43498/how-can-i-use-a-12-v-input-on-a-digital-arduino-pin

or go ssr. assuming, though ssr's more expensive, easier implement since i'll purchase ssr need, place in circuit.

thus question, elegant solution, more robust solution?

i'll have figure out ssr go if do, sure won't difficult. although, i'm not sure meant all. merely found 1 based purely on price @ glance , 15v.
http://za.rs-online.com/web/p/solid-state-relays/6996496/?origin=psf_428032|fp&cm_sp=featureproducts-_-featuredproductscontent-_-6996496

thank kindly experienced advice.

the automotive electrical environment extremely harsh , electronics, including inputs optoisolators , ssrs, need protected against transients can exceed 100 v.

i've attached all-around circuit. (the surge zener on input costs $0.25 , can pass 100 amperes). see links in following thread more information. http://forum.arduino.cc/index.php?topic=219048.0

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