Neither - it's a metaphor for lost times of our youth, the juxtaposition of a Deadhead sticker - a symbol of hippy freedom, on a Cadillac - a symbol of matrurity and success. That's what the song is about - don't look back, you can never look back .....
well i always thought it was about b4 summer the man in the song is asking if their love will last through the season. Incidently the boys of summer are baseball players the game being the main US past-time in the summer changing in the fall to gridiron.
Personally I think it is about after summer - 'the sun goes down alone' contrasting with when it has recently been going down with lots of people. there are other hints. the man lives in a seaside, or lakeside rather, holiday resort and his love has been stolen away by a 'boy of summer' who is just using her for a fling and the man is saying 'my love will last forever' as opposed to his.
i wonder if Don visits answerbank... the only way we'll ever know for sure!
a place you could try for an answer isthe official fan club
www.donhenley.com maybe you could get a response
from the man himself....personally i think its before summer