i know. i was actually at the junkyard looking for parts for another car and i just decided to stroll by the corollas and see if i found anything. i saw one with a spoiler and i was like yeah i cant leave without this. lol. and then guess what? when i turn around another corolla had the orange/red center trunk garnish which i dont see too ofter so i took that too. i was lucky that day. i didnt really have dough so i didnt take the trunk, only the spoiler.
I just pulled the whole trunk.... and the brake light for the spoiler plugs directly into where the rear deck light plugs into... Grab that small plastic plate to cover up the hole when you pull out the deck lamp.
I have a black spoiler (loose) if anyone wants it.
I have two loose spoilers actually. A dealer one and an aftermarket. Mounting would be easy, just drill holes and screw it together. The hard parts are measuring where to drill holes and how to wire the brake light.
When I eventually get around to painting and mounting my dealer option spoiler, my plan was to measure the width of the trunk lid; then cross reference that with the width between the spoiler's mounting points. At that point you can use simple math to determine the distance from the edge of the trunk lid you need to drill holes. I would also use some kind of paint on the holes after drilling them to prevent rust.
Your other option is to go back to the junkyard and measure the trunk lid of the car you pulled it from since it already has holes drilled.
For wiring, your options would be to either splice into the brake light wiring for the main tail lights because those wires should be accessible in the trunk area; or, run a wire up to the 3rd brake light where you could either use a pigtail in the bulb connector (since the spoiler light would legally be able to replace the 3rd brake light) or splice into the wiring for the 3rd brake light and retain the bulb.
Maybe you could make a template for the bolt holes out of cardboard.
Or you could do what I did when I was 16. I saw a spoiler that was perfect for my 240sx at the junkyard. I gave up trying to get holes drilled in the correct locations to properly bolt the spoiler on, so I glued it. I covered the underside of each end with gorilla glue and eyeballed it with the help of a friend. Placement turned out great. When the glue dried I tried to rip the spoiler off and couldn't so I figured it was on there good enough. Stayed in place years until I sold the car and as a bonus, no holes in the trunk so no chance of water leaks.