Ours is a canvas sided model so it might be different. I'm thinking that it would probably be somewhere near the entry door honestly, but I can't find a picture anywhere.Hello, fellow 89 Mustang owner! Hard-sided model?
I'm not 100% certain of my recollection, but my first thought is that it might have been on the drivers side support pillar that you lock in when setting up, and connect the lights at the top. I'd be glad to take pictures when it gets warm enough for me to pull my mustang hardwall out of storage and open it up.