This has been cracked.
HUGE SPOILER
You have to get 100% completion, then use the buggy you get for collecting all the spaceship parts to go to the top of the mountain at night and honk 3 times or something like that. Click here if you want to see what happens.
WOOOOOOOOOOOOOOOOW MAAAN
GODSTAR!!
I expect DLC hehe
Have you seen the pic on the wall inside the cable car station?
No!! gimme! gimme!