I barely ever visited junkyards in a decade playing. Only other time that pops into my mind is when I was playing a werewolf and he wished to visit a junkyard on a date. Long story short, I ventured i...
No personal experience here either, but I've read you can call the repairman and he'll (she'll) repair anything from the junkyard while at your home. Otherwise, I think sims have to be level 10 in inventing to repair things themselves.