Hello everyone - I have just added a spoiler to my '91 Legacy and I now want to wire in the brake/stop light. Is there an existing (hidden) cable for this that I can plug directly into? If so where can I find it? Or will I have to splice into one of the regular brake lights?

If this is a sedan, open the trunk and look under the rear deck (rear window) for a two pin plug that goes to the high mount brake light. You can wire into that.

Great! Thank you so much!

there could also be a trailer wiring harness? not sure on the older models.

but likely still require a splice.