Chuck also liked days like this: unlimited visibility, no wind, perfect teaching conditions. Nevertheless, the Seneca rocked slightly as she overcorrected.
“You’re losing your happy thoughts. Don’t do that. Come to one-twenty. Let’s go out Route 119. And drop on down to nine hundred.”
Q 1: What's the meaning of “You’re losing your happy thoughts."? For some reason (i don't know why) the driver needs to thinking positively to be able to drive.
Q 2: In this case, can I replace 'drop on down' with 'drop down'? What's the difference between 'drop on down' and 'drop down'? Yes you can, drop on down is probably just a local mannerism. Difference, nothing that I can think of , the "on" adds nothing to the meaning and is redundant.
Q 3: What's the meaning of “go out Route 119"? Tricky as it is poor English, but I suspect they have a road called "119" logically it should be "go out ON Route 119", here "on" is not redundant.
Q 4: In this case, Let’s go out Route 119. == Let's go [out Route 119]? see Q3
I think your subject matter is really written in a vernacular and while it is a form of English it is not helping with your understanding. This is a bit like trying to understand technica English before you understand normal English. Brave but maybe not helpful.