How do I get a heart with Scrap?
First, make sure you suggest him to hug for greeting and say goodbye instead of kissing. Because if you choose a wrong choice, you can't get his 1st heart (in this update).
Second, after complete some part of The rat's problem event (until it tell you 'The rest of this event is work in process' or something like that) bring Tao with you to explore a junkyard and take a visit to Scrap's place. I'm not so sure about this part but if I remember correctly, at the first time Scrap will hide from you and Tao. Then you should talk with Scrap alone and tell him about Tao.
Third, bring Tao with you while explore a junkyard and take a visit to Scrap's place again. There will have a tea time scene or something like that. At the end of this scene you will gain his 1st heart.